Outliner: Make use of new C++ based functional iterators

(Not meant to cause user visible changes.)

Makes use of the new iterators introduced in the previous commit. Some
benefits:
- Shorter, simpler, easier to read & understand
- Deduplicates logic
- Centralizes iteration logic, making it easier to replace tree storage
  (as planned), see previous commit.
- Avoids having to pass (sub-)tree to iterate around (often redundant
  since it's just `SpaceOutliner.tree`, even though `SpaceOutliner` is
  already passed).
- Function arguments that are only passed to the recursive call are
  recognized as unused (found and removed a few).

Also does some general cleanups while refactoring the code for the
iterators. Use `const`, use references (signals null is not expected),
early-exit (see 16fd5fa656af), remove redundant arguments, etc.

9 files changed, 416 insertions, 614 deletions

diff --git a/source/blender/editors/space_outliner/outliner_context.cc b/source/blender/editors/space_outliner/outliner_context.cc
index d07b6641836..1a804cb58b8 100644
--- a/source/blender/editors/space_outliner/outliner_context.cc
+++ b/source/blender/editors/space_outliner/outliner_context.cc
@@ -12,23 +12,25 @@
 #include "DNA_space_types.h"
 
 #include "outliner_intern.hh"
+#include "tree/tree_iterator.hh"
 
-static void outliner_context_selected_ids_recursive(const ListBase *subtree,
+using namespace blender::ed::outliner;
+
+static void outliner_context_selected_ids_recursive(const SpaceOutliner &space_outliner,
                                                     bContextDataResult *result)
 {
-  LISTBASE_FOREACH (const TreeElement *, te, subtree) {
+  tree_iterator::all(space_outliner, [&](const TreeElement *te) {
     const TreeStoreElem *tse = TREESTORE(te);
     if ((tse->flag & TSE_SELECTED) && (ELEM(tse->type, TSE_SOME_ID, TSE_LAYER_COLLECTION))) {
       CTX_data_id_list_add(result, tse->id);
     }
-    outliner_context_selected_ids_recursive(&te->subtree, result);
-  }
+  });
 }
 
 static void outliner_context_selected_ids(const SpaceOutliner *space_outliner,
                                           bContextDataResult *result)
 {
-  outliner_context_selected_ids_recursive(&space_outliner->tree, result);
+  outliner_context_selected_ids_recursive(*space_outliner, result);
   CTX_data_type_set(result, CTX_DATA_TYPE_COLLECTION);
 }
 
diff --git a/source/blender/editors/space_outliner/outliner_dragdrop.cc b/source/blender/editors/space_outliner/outliner_dragdrop.cc
index a22ce9d3d24..e20958c1b1e 100644
--- a/source/blender/editors/space_outliner/outliner_dragdrop.cc
+++ b/source/blender/editors/space_outliner/outliner_dragdrop.cc
@@ -316,7 +316,7 @@ static bool parent_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event)
 {
   SpaceOutliner *space_outliner = CTX_wm_space_outliner(C);
 
-  bool changed = outliner_flag_set(&space_outliner->tree, TSE_DRAG_ANY, false);
+  bool changed = outliner_flag_set(*space_outliner, TSE_DRAG_ANY, false);
   if (changed) {
     ED_region_tag_redraw_no_rebuild(CTX_wm_region(C));
   }
@@ -847,8 +847,7 @@ static bool datastack_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event)
 
   SpaceOutliner *space_outliner = CTX_wm_space_outliner(C);
   ARegion *region = CTX_wm_region(C);
-  bool changed = outliner_flag_set(
-      &space_outliner->tree, TSE_HIGHLIGHTED_ANY | TSE_DRAG_ANY, false);
+  bool changed = outliner_flag_set(*space_outliner, TSE_HIGHLIGHTED_ANY | TSE_DRAG_ANY, false);
 
   StackDropData *drop_data = reinterpret_cast<StackDropData *>(drag->poin);
   if (!drop_data) {
@@ -1195,8 +1194,7 @@ static bool collection_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event
 {
   SpaceOutliner *space_outliner = CTX_wm_space_outliner(C);
   ARegion *region = CTX_wm_region(C);
-  bool changed = outliner_flag_set(
-      &space_outliner->tree, TSE_HIGHLIGHTED_ANY | TSE_DRAG_ANY, false);
+  bool changed = outliner_flag_set(*space_outliner, TSE_HIGHLIGHTED_ANY | TSE_DRAG_ANY, false);
 
   CollectionDrop data;
   if (((event->modifier & KM_SHIFT) == 0) &&
@@ -1461,7 +1459,7 @@ static int outliner_item_drag_drop_invoke(bContext *C,
 
     /* Only drag element under mouse if it was not selected before. */
     if ((tselem->flag & TSE_SELECTED) == 0) {
-      outliner_flag_set(&space_outliner->tree, TSE_SELECTED, 0);
+      outliner_flag_set(*space_outliner, TSE_SELECTED, 0);
       tselem->flag |= TSE_SELECTED;
     }
 
diff --git a/source/blender/editors/space_outliner/outliner_draw.cc b/source/blender/editors/space_outliner/outliner_draw.cc
index 92353c02d3f..753de83a10d 100644
--- a/source/blender/editors/space_outliner/outliner_draw.cc
+++ b/source/blender/editors/space_outliner/outliner_draw.cc
@@ -71,6 +71,7 @@
 #include "tree/tree_element_id.hh"
 #include "tree/tree_element_overrides.hh"
 #include "tree/tree_element_rna.hh"
+#include "tree/tree_iterator.hh"
 
 using namespace blender;
 using namespace blender::ed::outliner;
@@ -1715,72 +1716,70 @@ static void outliner_draw_restrictbuts(uiBlock *block,
 }
 
 static void outliner_draw_userbuts(uiBlock *block,
-                                   ARegion *region,
-                                   SpaceOutliner *space_outliner,
-                                   ListBase *lb)
+                                   const ARegion *region,
+                                   const SpaceOutliner *space_outliner)
 {
+  tree_iterator::all_open(*space_outliner, [&](const TreeElement *te) {
+    if (!outliner_is_element_in_view(te, &region->v2d)) {
+      return;
+    }
 
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
-    TreeStoreElem *tselem = TREESTORE(te);
-    if (outliner_is_element_in_view(te, &region->v2d)) {
-      if (tselem->type == TSE_SOME_ID) {
-        uiBut *bt;
-        ID *id = tselem->id;
-        const char *tip = nullptr;
-        char buf[16] = "";
-        int but_flag = UI_BUT_DRAG_LOCK;
+    const TreeStoreElem *tselem = TREESTORE(te);
+    if (tselem->type != TSE_SOME_ID) {
+      return;
+    }
 
-        if (ID_IS_LINKED(id)) {
-          but_flag |= UI_BUT_DISABLED;
-        }
+    uiBut *bt;
+    ID *id = tselem->id;
+    const char *tip = nullptr;
+    char buf[16] = "";
+    int but_flag = UI_BUT_DRAG_LOCK;
 
-        BLI_str_format_int_grouped(buf, id->us);
-        bt = uiDefBut(block,
-                      UI_BTYPE_BUT,
-                      1,
-                      buf,
-                      (int)(region->v2d.cur.xmax - OL_TOG_USER_BUTS_USERS),
-                      te->ys,
-                      UI_UNIT_X,
-                      UI_UNIT_Y,
-                      nullptr,
-                      0.0,
-                      0.0,
-                      0,
-                      0,
-                      TIP_("Number of users of this data-block"));
-        UI_but_flag_enable(bt, but_flag);
-
-        if (id->flag & LIB_FAKEUSER) {
-          tip = TIP_("Data-block will be retained using a fake user");
-        }
-        else {
-          tip = TIP_("Data-block has no users and will be deleted");
-        }
-        bt = uiDefIconButBitS(block,
-                              UI_BTYPE_ICON_TOGGLE,
-                              LIB_FAKEUSER,
-                              1,
-                              ICON_FAKE_USER_OFF,
-                              (int)(region->v2d.cur.xmax - OL_TOG_USER_BUTS_STATUS),
-                              te->ys,
-                              UI_UNIT_X,
-                              UI_UNIT_Y,
-                              &id->flag,
-                              0,
-                              0,
-                              0,
-                              0,
-                              tip);
-        UI_but_func_set(bt, restrictbutton_id_user_toggle, id, nullptr);
-        UI_but_flag_enable(bt, but_flag);
-      }
+    if (ID_IS_LINKED(id)) {
+      but_flag |= UI_BUT_DISABLED;
     }
 
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_draw_userbuts(block, region, space_outliner, &te->subtree);
+    BLI_str_format_int_grouped(buf, id->us);
+    bt = uiDefBut(block,
+                  UI_BTYPE_BUT,
+                  1,
+                  buf,
+                  (int)(region->v2d.cur.xmax - OL_TOG_USER_BUTS_USERS),
+                  te->ys,
+                  UI_UNIT_X,
+                  UI_UNIT_Y,
+                  nullptr,
+                  0.0,
+                  0.0,
+                  0,
+                  0,
+                  TIP_("Number of users of this data-block"));
+    UI_but_flag_enable(bt, but_flag);
+
+    if (id->flag & LIB_FAKEUSER) {
+      tip = TIP_("Data-block will be retained using a fake user");
     }
-  }
+    else {
+      tip = TIP_("Data-block has no users and will be deleted");
+    }
+    bt = uiDefIconButBitS(block,
+                          UI_BTYPE_ICON_TOGGLE,
+                          LIB_FAKEUSER,
+                          1,
+                          ICON_FAKE_USER_OFF,
+                          (int)(region->v2d.cur.xmax - OL_TOG_USER_BUTS_STATUS),
+                          te->ys,
+                          UI_UNIT_X,
+                          UI_UNIT_Y,
+                          &id->flag,
+                          0,
+                          0,
+                          0,
+                          0,
+                          tip);
+    UI_but_func_set(bt, restrictbutton_id_user_toggle, id, nullptr);
+    UI_but_flag_enable(bt, but_flag);
+  });
 }
 
 static void outliner_draw_overrides_rna_buts(uiBlock *block,
@@ -1941,81 +1940,82 @@ static void outliner_draw_separator(ARegion *region, const int x)
   immUnbindProgram();
 }
 
-static void outliner_draw_rnabuts(
-    uiBlock *block, ARegion *region, SpaceOutliner *space_outliner, int sizex, ListBase *lb)
+static void outliner_draw_rnabuts(uiBlock *block,
+                                  ARegion *region,
+                                  SpaceOutliner *space_outliner,
+                                  int sizex)
 {
   PointerRNA ptr;
   PropertyRNA *prop;
 
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
-    if (outliner_is_element_in_view(te, &region->v2d)) {
-      if (TreeElementRNAProperty *te_rna_prop = tree_element_cast<TreeElementRNAProperty>(te)) {
-        ptr = te_rna_prop->getPointerRNA();
-        prop = te_rna_prop->getPropertyRNA();
-
-        if (!TSELEM_OPEN(tselem, space_outliner)) {
-          if (RNA_property_type(prop) == PROP_POINTER) {
-            uiBut *but = uiDefAutoButR(block,
-                                       &ptr,
-                                       prop,
-                                       -1,
-                                       "",
-                                       ICON_NONE,
-                                       sizex,
-                                       te->ys,
-                                       OL_RNA_COL_SIZEX,
-                                       UI_UNIT_Y - 1);
-            UI_but_flag_enable(but, UI_BUT_DISABLED);
-          }
-          else if (RNA_property_type(prop) == PROP_ENUM) {
-            uiDefAutoButR(block,
-                          &ptr,
-                          prop,
-                          -1,
-                          nullptr,
-                          ICON_NONE,
-                          sizex,
-                          te->ys,
-                          OL_RNA_COL_SIZEX,
-                          UI_UNIT_Y - 1);
-          }
-          else {
-            uiDefAutoButR(block,
-                          &ptr,
-                          prop,
-                          -1,
-                          "",
-                          ICON_NONE,
-                          sizex,
-                          te->ys,
-                          OL_RNA_COL_SIZEX,
-                          UI_UNIT_Y - 1);
-          }
+
+    if (!outliner_is_element_in_view(te, &region->v2d)) {
+      return;
+    }
+
+    if (TreeElementRNAProperty *te_rna_prop = tree_element_cast<TreeElementRNAProperty>(te)) {
+      ptr = te_rna_prop->getPointerRNA();
+      prop = te_rna_prop->getPropertyRNA();
+
+      if (!TSELEM_OPEN(tselem, space_outliner)) {
+        if (RNA_property_type(prop) == PROP_POINTER) {
+          uiBut *but = uiDefAutoButR(block,
+                                     &ptr,
+                                     prop,
+                                     -1,
+                                     "",
+                                     ICON_NONE,
+                                     sizex,
+                                     te->ys,
+                                     OL_RNA_COL_SIZEX,
+                                     UI_UNIT_Y - 1);
+          UI_but_flag_enable(but, UI_BUT_DISABLED);
+        }
+        else if (RNA_property_type(prop) == PROP_ENUM) {
+          uiDefAutoButR(block,
+                        &ptr,
+                        prop,
+                        -1,
+                        nullptr,
+                        ICON_NONE,
+                        sizex,
+                        te->ys,
+                        OL_RNA_COL_SIZEX,
+                        UI_UNIT_Y - 1);
+        }
+        else {
+          uiDefAutoButR(block,
+                        &ptr,
+                        prop,
+                        -1,
+                        "",
+                        ICON_NONE,
+                        sizex,
+                        te->ys,
+                        OL_RNA_COL_SIZEX,
+                        UI_UNIT_Y - 1);
         }
-      }
-      else if (TreeElementRNAArrayElement *te_rna_array_elem =
-                   tree_element_cast<TreeElementRNAArrayElement>(te)) {
-        ptr = te_rna_array_elem->getPointerRNA();
-        prop = te_rna_array_elem->getPropertyRNA();
-
-        uiDefAutoButR(block,
-                      &ptr,
-                      prop,
-                      te->index,
-                      "",
-                      ICON_NONE,
-                      sizex,
-                      te->ys,
-                      OL_RNA_COL_SIZEX,
-                      UI_UNIT_Y - 1);
       }
     }
+    else if (TreeElementRNAArrayElement *te_rna_array_elem =
+                 tree_element_cast<TreeElementRNAArrayElement>(te)) {
+      ptr = te_rna_array_elem->getPointerRNA();
+      prop = te_rna_array_elem->getPropertyRNA();
 
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_draw_rnabuts(block, region, space_outliner, sizex, &te->subtree);
+      uiDefAutoButR(block,
+                    &ptr,
+                    prop,
+                    te->index,
+                    "",
+                    ICON_NONE,
+                    sizex,
+                    te->ys,
+                    OL_RNA_COL_SIZEX,
+                    UI_UNIT_Y - 1);
     }
-  }
+  });
 }
 
 static void outliner_buttons(const bContext *C,
@@ -2101,9 +2101,9 @@ static void outliner_mode_toggle_fn(bContext *C, void *tselem_poin, void *UNUSED
 static void outliner_draw_mode_column_toggle(uiBlock *block,
                                              TreeViewContext *tvc,
                                              TreeElement *te,
-                                             TreeStoreElem *tselem,
                                              const bool lock_object_modes)
 {
+  TreeStoreElem *tselem = TREESTORE(te);
   if ((tselem->type != TSE_SOME_ID) || (te->idcode != ID_OB)) {
     return;
   }
@@ -2174,26 +2174,17 @@ static void outliner_draw_mode_column_toggle(uiBlock *block,
   }
 }
 
-static void outliner_draw_mode_column(const bContext *C,
-                                      uiBlock *block,
+static void outliner_draw_mode_column(uiBlock *block,
                                       TreeViewContext *tvc,
-                                      SpaceOutliner *space_outliner,
-                                      ListBase *tree)
+                                      SpaceOutliner *space_outliner)
 {
-  TreeStoreElem *tselem;
   const bool lock_object_modes = tvc->scene->toolsettings->object_flag & SCE_OBJECT_MODE_LOCK;
 
-  LISTBASE_FOREACH (TreeElement *, te, tree) {
-    tselem = TREESTORE(te);
-
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     if (tvc->obact && tvc->obact->mode != OB_MODE_OBJECT) {
-      outliner_draw_mode_column_toggle(block, tvc, te, tselem, lock_object_modes);
-    }
-
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_draw_mode_column(C, block, tvc, space_outliner, &te->subtree);
+      outliner_draw_mode_column_toggle(block, tvc, te, lock_object_modes);
     }
-  }
+  });
 }
 
 static StringRefNull outliner_draw_get_warning_tree_element_subtree(const TreeElement *parent_te)
@@ -2235,7 +2226,7 @@ static StringRefNull outliner_draw_get_warning_tree_element(const SpaceOutliner
 }
 
 static void outliner_draw_warning_tree_element(uiBlock *block,
-                                               SpaceOutliner *space_outliner,
+                                               const SpaceOutliner *space_outliner,
                                                StringRefNull warning_msg,
                                                const bool use_mode_column,
                                                const int te_ys)
@@ -2264,13 +2255,11 @@ static void outliner_draw_warning_tree_element(uiBlock *block,
   UI_but_flag_disable(but, UI_BUT_UNDO);
 }
 
-static void outliner_draw_warning_column(const bContext *C,
-                                         uiBlock *block,
-                                         SpaceOutliner *space_outliner,
-                                         const bool use_mode_column,
-                                         ListBase *tree)
+static void outliner_draw_warning_column(uiBlock *block,
+                                         const SpaceOutliner *space_outliner,
+                                         const bool use_mode_column)
 {
-  LISTBASE_FOREACH (TreeElement *, te, tree) {
+  tree_iterator::all_open(*space_outliner, [&](const TreeElement *te) {
     /* Get warning for this element, or if there is none and the element is collapsed, the first
      * warning in the collapsed sub-tree. */
     StringRefNull warning_msg = outliner_draw_get_warning_tree_element(*space_outliner, te);
@@ -2279,11 +2268,7 @@ static void outliner_draw_warning_column(const bContext *C,
       outliner_draw_warning_tree_element(
           block, space_outliner, warning_msg, use_mode_column, te->ys);
     }
-
-    if (TSELEM_OPEN(te->store_elem, space_outliner)) {
-      outliner_draw_warning_column(C, block, space_outliner, use_mode_column, &te->subtree);
-    }
-  }
+  });
 }
 
 /** \} */
@@ -3164,18 +3149,16 @@ static void outliner_draw_iconrow(bContext *C,
 }
 
 /* closed tree element */
-static void outliner_set_coord_tree_element(TreeElement *te, int startx, int starty)
+static void outliner_set_subtree_coords(const TreeElement *te)
 {
-  /* closed items may be displayed in row of parent, don't change their coordinate! */
-  if ((te->flag & TE_ICONROW) == 0 && (te->flag & TE_ICONROW_MERGED) == 0) {
-    te->xs = 0;
-    te->ys = 0;
-    te->xend = 0;
-  }
-
-  LISTBASE_FOREACH (TreeElement *, ten, &te->subtree) {
-    outliner_set_coord_tree_element(ten, startx + UI_UNIT_X, starty);
-  }
+  tree_iterator::all(te->subtree, [&](TreeElement *te) {
+    /* closed items may be displayed in row of parent, don't change their coordinate! */
+    if ((te->flag & TE_ICONROW) == 0 && (te->flag & TE_ICONROW_MERGED) == 0) {
+      te->xs = 0;
+      te->ys = 0;
+      te->xend = 0;
+    }
+  });
 }
 
 static bool element_should_draw_faded(const TreeViewContext *tvc,
@@ -3427,10 +3410,7 @@ static void outliner_draw_tree_element(bContext *C,
     }
   }
   else {
-    LISTBASE_FOREACH (TreeElement *, ten, &te->subtree) {
-      outliner_set_coord_tree_element(ten, startx, *starty);
-    }
-
+    outliner_set_subtree_coords(te);
     *starty -= UI_UNIT_Y;
   }
 }
@@ -3586,22 +3566,21 @@ static void outliner_draw_struct_marks(ARegion *region,
   }
 }
 
-static void outliner_draw_highlights_recursive(uint pos,
-                                               const ARegion *region,
-                                               const SpaceOutliner *space_outliner,
-                                               const ListBase *lb,
-                                               const float col_selection[4],
-                                               const float col_active[4],
-                                               const float col_highlight[4],
-                                               const float col_searchmatch[4],
-                                               int start_x,
-                                               int *io_start_y)
+static void outliner_draw_highlights(uint pos,
+                                     const ARegion *region,
+                                     const SpaceOutliner *space_outliner,
+                                     const float col_selection[4],
+                                     const float col_active[4],
+                                     const float col_highlight[4],
+                                     const float col_searchmatch[4],
+                                     int start_x,
+                                     int *io_start_y)
 {
   const bool is_searching = (SEARCHING_OUTLINER(space_outliner) ||
                              (space_outliner->outlinevis == SO_DATA_API &&
                               space_outliner->search_string[0] != 0));
 
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all_open(*space_outliner, [&](const TreeElement *te) {
     const TreeStoreElem *tselem = TREESTORE(te);
     const int start_y = *io_start_y;
 
@@ -3657,19 +3636,7 @@ static void outliner_draw_highlights_recursive(uint pos,
     }
 
     *io_start_y -= UI_UNIT_Y;
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_draw_highlights_recursive(pos,
-                                         region,
-                                         space_outliner,
-                                         &te->subtree,
-                                         col_selection,
-                                         col_active,
-                                         col_highlight,
-                                         col_searchmatch,
-                                         start_x + UI_UNIT_X,
-                                         io_start_y);
-    }
-  }
+  });
 }
 
 static void outliner_draw_highlights(ARegion *region,
@@ -3691,16 +3658,15 @@ static void outliner_draw_highlights(ARegion *region,
   GPUVertFormat *format = immVertexFormat();
   uint pos = GPU_vertformat_attr_add(format, "pos", GPU_COMP_I32, 2, GPU_FETCH_INT_TO_FLOAT);
   immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
-  outliner_draw_highlights_recursive(pos,
-                                     region,
-                                     space_outliner,
-                                     &space_outliner->tree,
-                                     col_selection,
-                                     col_active,
-                                     col_highlight,
-                                     col_searchmatch,
-                                     startx,
-                                     starty);
+  outliner_draw_highlights(pos,
+                           region,
+                           space_outliner,
+                           col_selection,
+                           col_active,
+                           col_highlight,
+                           col_searchmatch,
+                           startx,
+                           starty);
   immUnbindProgram();
   GPU_blend(GPU_BLEND_NONE);
 }
@@ -3924,12 +3890,12 @@ void draw_outliner(const bContext *C)
     outliner_draw_separator(region, buttons_start_x + OL_RNA_COL_SIZEX);
 
     UI_block_emboss_set(block, UI_EMBOSS);
-    outliner_draw_rnabuts(block, region, space_outliner, buttons_start_x, &space_outliner->tree);
+    outliner_draw_rnabuts(block, region, space_outliner, buttons_start_x);
     UI_block_emboss_set(block, UI_EMBOSS_NONE_OR_STATUS);
   }
   else if (space_outliner->outlinevis == SO_ID_ORPHANS) {
     /* draw user toggle columns */
-    outliner_draw_userbuts(block, region, space_outliner, &space_outliner->tree);
+    outliner_draw_userbuts(block, region, space_outliner);
   }
   else if (space_outliner->outlinevis == SO_OVERRIDES_LIBRARY) {
     const int x = region->v2d.cur.xmax - right_column_width;
@@ -3960,12 +3926,12 @@ void draw_outliner(const bContext *C)
 
   /* Draw mode icons */
   if (use_mode_column) {
-    outliner_draw_mode_column(C, block, &tvc, space_outliner, &space_outliner->tree);
+    outliner_draw_mode_column(block, &tvc, space_outliner);
   }
 
   /* Draw warning icons */
   if (use_warning_column) {
-    outliner_draw_warning_column(C, block, space_outliner, use_mode_column, &space_outliner->tree);
+    outliner_draw_warning_column(block, space_outliner, use_mode_column);
   }
 
   UI_block_emboss_set(block, UI_EMBOSS);
diff --git a/source/blender/editors/space_outliner/outliner_edit.cc b/source/blender/editors/space_outliner/outliner_edit.cc
index 1de45b0ec96..c4a9398a5f7 100644
--- a/source/blender/editors/space_outliner/outliner_edit.cc
+++ b/source/blender/editors/space_outliner/outliner_edit.cc
@@ -60,6 +60,7 @@
 
 #include "outliner_intern.hh"
 #include "tree/tree_element_rna.hh"
+#include "tree/tree_iterator.hh"
 
 using namespace blender::ed::outliner;
 
@@ -107,7 +108,7 @@ static int outliner_highlight_update(bContext *C, wmOperator *UNUSED(op), const
   if (!hovered_te || !is_over_icon || !(hovered_te->store_elem->flag & TSE_HIGHLIGHTED) ||
       !(icon_te->store_elem->flag & TSE_HIGHLIGHTED_ICON)) {
     /* Clear highlights when nothing is hovered or when a new item is hovered. */
-    changed = outliner_flag_set(&space_outliner->tree, TSE_HIGHLIGHTED_ANY | TSE_DRAG_ANY, false);
+    changed = outliner_flag_set(*space_outliner, TSE_HIGHLIGHTED_ANY | TSE_DRAG_ANY, false);
     if (hovered_te) {
       hovered_te->store_elem->flag |= TSE_HIGHLIGHTED;
       changed = true;
@@ -167,7 +168,7 @@ void outliner_item_openclose(SpaceOutliner *space_outliner,
   }
 
   if (toggle_all) {
-    outliner_flag_set(&te->subtree, TSE_CLOSED, !open);
+    outliner_flag_set(te->subtree, TSE_CLOSED, !open);
   }
 }
 
@@ -1077,11 +1078,16 @@ int outliner_flag_is_any_test(ListBase *lb, short flag, const int curlevel)
   return 0;
 }
 
-bool outliner_flag_set(ListBase *lb, short flag, short set)
+bool outliner_flag_set(const SpaceOutliner &space_outliner, const short flag, const short set)
+{
+  return outliner_flag_set(space_outliner.tree, flag, set);
+}
+
+bool outliner_flag_set(const ListBase &lb, const short flag, const short set)
 {
   bool changed = false;
 
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all(lb, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
     bool has_flag = (tselem->flag & flag);
     if (set == 0) {
@@ -1094,21 +1100,24 @@ bool outliner_flag_set(ListBase *lb, short flag, short set)
       tselem->flag |= flag;
       changed = true;
     }
-    changed |= outliner_flag_set(&te->subtree, flag, set);
-  }
+  });
 
   return changed;
 }
 
-bool outliner_flag_flip(ListBase *lb, short flag)
+bool outliner_flag_flip(const SpaceOutliner &space_outliner, const short flag)
+{
+  return outliner_flag_flip(space_outliner.tree, flag);
+}
+
+bool outliner_flag_flip(const ListBase &lb, const short flag)
 {
   bool changed = false;
 
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all(lb, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
     tselem->flag ^= flag;
-    changed |= outliner_flag_flip(&te->subtree, flag);
-  }
+  });
 
   return changed;
 }
@@ -1125,10 +1134,10 @@ static int outliner_toggle_expanded_exec(bContext *C, wmOperator *UNUSED(op))
   ARegion *region = CTX_wm_region(C);
 
   if (outliner_flag_is_any_test(&space_outliner->tree, TSE_CLOSED, 1)) {
-    outliner_flag_set(&space_outliner->tree, TSE_CLOSED, 0);
+    outliner_flag_set(*space_outliner, TSE_CLOSED, 0);
   }
   else {
-    outliner_flag_set(&space_outliner->tree, TSE_CLOSED, 1);
+    outliner_flag_set(*space_outliner, TSE_CLOSED, 1);
   }
 
   ED_region_tag_redraw(region);
@@ -1169,13 +1178,13 @@ static int outliner_select_all_exec(bContext *C, wmOperator *op)
 
   switch (action) {
     case SEL_SELECT:
-      outliner_flag_set(&space_outliner->tree, TSE_SELECTED, 1);
+      outliner_flag_set(*space_outliner, TSE_SELECTED, 1);
       break;
     case SEL_DESELECT:
-      outliner_flag_set(&space_outliner->tree, TSE_SELECTED, 0);
+      outliner_flag_set(*space_outliner, TSE_SELECTED, 0);
       break;
     case SEL_INVERT:
-      outliner_flag_flip(&space_outliner->tree, TSE_SELECTED);
+      outliner_flag_flip(*space_outliner, TSE_SELECTED);
       break;
   }
 
@@ -1211,32 +1220,16 @@ void OUTLINER_OT_select_all(wmOperatorType *ot)
 /** \name View Show Active (Outliner) Operator
  * \{ */
 
-static void outliner_set_coordinates_element_recursive(SpaceOutliner *space_outliner,
-                                                       TreeElement *te,
-                                                       int startx,
-                                                       int *starty)
-{
-  TreeStoreElem *tselem = TREESTORE(te);
-
-  /* store coord and continue, we need coordinates for elements outside view too */
-  te->xs = (float)startx;
-  te->ys = (float)(*starty);
-  *starty -= UI_UNIT_Y;
-
-  if (TSELEM_OPEN(tselem, space_outliner)) {
-    LISTBASE_FOREACH (TreeElement *, ten, &te->subtree) {
-      outliner_set_coordinates_element_recursive(space_outliner, ten, startx + UI_UNIT_X, starty);
-    }
-  }
-}
-
-void outliner_set_coordinates(ARegion *region, SpaceOutliner *space_outliner)
+void outliner_set_coordinates(const ARegion *region, const SpaceOutliner *space_outliner)
 {
   int starty = (int)(region->v2d.tot.ymax) - UI_UNIT_Y;
 
-  LISTBASE_FOREACH (TreeElement *, te, &space_outliner->tree) {
-    outliner_set_coordinates_element_recursive(space_outliner, te, 0, &starty);
-  }
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
+    /* store coord and continue, we need coordinates for elements outside view too */
+    te->xs = 0;
+    te->ys = (float)starty;
+    starty -= UI_UNIT_Y;
+  });
 }
 
 /* return 1 when levels were opened */
@@ -1624,11 +1617,11 @@ static int subtree_has_objects(ListBase *lb)
   return 0;
 }
 
-/* recursive helper function for Show Hierarchy operator */
-static void tree_element_show_hierarchy(Scene *scene, SpaceOutliner *space_outliner, ListBase *lb)
+/* Helper function for Show Hierarchy operator */
+static void tree_element_show_hierarchy(Scene *scene, SpaceOutliner *space_outliner)
 {
   /* open all object elems, close others */
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
 
     if (ELEM(tselem->type,
@@ -1656,11 +1649,7 @@ static void tree_element_show_hierarchy(Scene *scene, SpaceOutliner *space_outli
     else {
       tselem->flag |= TSE_CLOSED;
     }
-
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      tree_element_show_hierarchy(scene, space_outliner, &te->subtree);
-    }
-  }
+  });
 }
 
 /* show entire object level hierarchy */
@@ -1671,7 +1660,7 @@ static int outliner_show_hierarchy_exec(bContext *C, wmOperator *UNUSED(op))
   Scene *scene = CTX_data_scene(C);
 
   /* recursively open/close levels */
-  tree_element_show_hierarchy(scene, space_outliner, &space_outliner->tree);
+  tree_element_show_hierarchy(scene, space_outliner);
 
   ED_region_tag_redraw(region);
 
@@ -1873,79 +1862,75 @@ enum {
   DRIVERS_EDITMODE_REMOVE,
 } /*eDrivers_EditModes*/;
 
-/* Recursively iterate over tree, finding and working on selected items */
+/* Iterate over tree, finding and working on selected items */
 static void do_outliner_drivers_editop(SpaceOutliner *space_outliner,
-                                       ListBase *tree,
                                        ReportList *reports,
                                        short mode)
 {
-  LISTBASE_FOREACH (TreeElement *, te, tree) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
 
     /* if item is selected, perform operation */
-    if (tselem->flag & TSE_SELECTED) {
-      ID *id = nullptr;
-      char *path = nullptr;
-      int array_index = 0;
-      short flag = 0;
-      short groupmode = KSP_GROUP_KSNAME;
-
-      TreeElementRNACommon *te_rna = tree_element_cast<TreeElementRNACommon>(te);
-      PointerRNA ptr = te_rna ? te_rna->getPointerRNA() : PointerRNA_NULL;
-      PropertyRNA *prop = te_rna ? te_rna->getPropertyRNA() : nullptr;
-
-      /* check if RNA-property described by this selected element is an animatable prop */
-      if (prop && RNA_property_animateable(&ptr, prop)) {
-        /* get id + path + index info from the selected element */
-        tree_element_to_path(te, tselem, &id, &path, &array_index, &flag, &groupmode);
-      }
+    if (!(tselem->flag & TSE_SELECTED)) {
+      return;
+    }
 
-      /* only if ID and path were set, should we perform any actions */
-      if (id && path) {
-        short dflags = CREATEDRIVER_WITH_DEFAULT_DVAR;
-        int arraylen = 1;
+    ID *id = nullptr;
+    char *path = nullptr;
+    int array_index = 0;
+    short flag = 0;
+    short groupmode = KSP_GROUP_KSNAME;
 
-        /* array checks */
-        if (flag & KSP_FLAG_WHOLE_ARRAY) {
-          /* entire array was selected, so add drivers for all */
-          arraylen = RNA_property_array_length(&ptr, prop);
-        }
-        else {
-          arraylen = array_index;
-        }
+    TreeElementRNACommon *te_rna = tree_element_cast<TreeElementRNACommon>(te);
+    PointerRNA ptr = te_rna ? te_rna->getPointerRNA() : PointerRNA_NULL;
+    PropertyRNA *prop = te_rna ? te_rna->getPropertyRNA() : nullptr;
 
-        /* we should do at least one step */
-        if (arraylen == array_index) {
-          arraylen++;
-        }
+    /* check if RNA-property described by this selected element is an animatable prop */
+    if (prop && RNA_property_animateable(&ptr, prop)) {
+      /* get id + path + index info from the selected element */
+      tree_element_to_path(te, tselem, &id, &path, &array_index, &flag, &groupmode);
+    }
 
-        /* for each array element we should affect, add driver */
-        for (; array_index < arraylen; array_index++) {
-          /* action depends on mode */
-          switch (mode) {
-            case DRIVERS_EDITMODE_ADD: {
-              /* add a new driver with the information obtained (only if valid) */
-              ANIM_add_driver(reports, id, path, array_index, dflags, DRIVER_TYPE_PYTHON);
-              break;
-            }
-            case DRIVERS_EDITMODE_REMOVE: {
-              /* remove driver matching the information obtained (only if valid) */
-              ANIM_remove_driver(reports, id, path, array_index, dflags);
-              break;
-            }
+    /* only if ID and path were set, should we perform any actions */
+    if (id && path) {
+      short dflags = CREATEDRIVER_WITH_DEFAULT_DVAR;
+      int arraylen = 1;
+
+      /* array checks */
+      if (flag & KSP_FLAG_WHOLE_ARRAY) {
+        /* entire array was selected, so add drivers for all */
+        arraylen = RNA_property_array_length(&ptr, prop);
+      }
+      else {
+        arraylen = array_index;
+      }
+
+      /* we should do at least one step */
+      if (arraylen == array_index) {
+        arraylen++;
+      }
+
+      /* for each array element we should affect, add driver */
+      for (; array_index < arraylen; array_index++) {
+        /* action depends on mode */
+        switch (mode) {
+          case DRIVERS_EDITMODE_ADD: {
+            /* add a new driver with the information obtained (only if valid) */
+            ANIM_add_driver(reports, id, path, array_index, dflags, DRIVER_TYPE_PYTHON);
+            break;
+          }
+          case DRIVERS_EDITMODE_REMOVE: {
+            /* remove driver matching the information obtained (only if valid) */
+            ANIM_remove_driver(reports, id, path, array_index, dflags);
+            break;
           }
         }
-
-        /* free path, since it had to be generated */
-        MEM_freeN(path);
       }
-    }
 
-    /* go over sub-tree */
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      do_outliner_drivers_editop(space_outliner, &te->subtree, reports, mode);
+      /* free path, since it had to be generated */
+      MEM_freeN(path);
     }
-  }
+  });
 }
 
 /** \} */
@@ -1964,8 +1949,7 @@ static int outliner_drivers_addsel_exec(bContext *C, wmOperator *op)
   }
 
   /* recursively go into tree, adding selected items */
-  do_outliner_drivers_editop(
-      space_outliner, &space_outliner->tree, op->reports, DRIVERS_EDITMODE_ADD);
+  do_outliner_drivers_editop(space_outliner, op->reports, DRIVERS_EDITMODE_ADD);
 
   /* send notifiers */
   WM_event_add_notifier(C, NC_ANIMATION | ND_FCURVES_ORDER, nullptr); /* XXX */
@@ -2004,8 +1988,7 @@ static int outliner_drivers_deletesel_exec(bContext *C, wmOperator *op)
   }
 
   /* recursively go into tree, adding selected items */
-  do_outliner_drivers_editop(
-      space_outliner, &space_outliner->tree, op->reports, DRIVERS_EDITMODE_REMOVE);
+  do_outliner_drivers_editop(space_outliner, op->reports, DRIVERS_EDITMODE_REMOVE);
 
   /* send notifiers */
   WM_event_add_notifier(C, ND_KEYS, nullptr); /* XXX */
@@ -2072,68 +2055,64 @@ static KeyingSet *verify_active_keyingset(Scene *scene, short add)
   return ks;
 }
 
-/* Recursively iterate over tree, finding and working on selected items */
+/* Iterate over tree, finding and working on selected items */
 static void do_outliner_keyingset_editop(SpaceOutliner *space_outliner,
                                          KeyingSet *ks,
-                                         ListBase *tree,
-                                         short mode)
+                                         const short mode)
 {
-  LISTBASE_FOREACH (TreeElement *, te, tree) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
 
     /* if item is selected, perform operation */
-    if (tselem->flag & TSE_SELECTED) {
-      ID *id = nullptr;
-      char *path = nullptr;
-      int array_index = 0;
-      short flag = 0;
-      short groupmode = KSP_GROUP_KSNAME;
-
-      /* check if RNA-property described by this selected element is an animatable prop */
-      const TreeElementRNACommon *te_rna = tree_element_cast<TreeElementRNACommon>(te);
-      PointerRNA ptr = te_rna->getPointerRNA();
-      if (te_rna && te_rna->getPropertyRNA() &&
-          RNA_property_animateable(&ptr, te_rna->getPropertyRNA())) {
-        /* get id + path + index info from the selected element */
-        tree_element_to_path(te, tselem, &id, &path, &array_index, &flag, &groupmode);
-      }
+    if (!(tselem->flag & TSE_SELECTED)) {
+      return;
+    }
 
-      /* only if ID and path were set, should we perform any actions */
-      if (id && path) {
-        /* action depends on mode */
-        switch (mode) {
-          case KEYINGSET_EDITMODE_ADD: {
-            /* add a new path with the information obtained (only if valid) */
-            /* TODO: what do we do with group name?
-             * for now, we don't supply one, and just let this use the KeyingSet name */
-            BKE_keyingset_add_path(ks, id, nullptr, path, array_index, flag, groupmode);
-            ks->active_path = BLI_listbase_count(&ks->paths);
-            break;
-          }
-          case KEYINGSET_EDITMODE_REMOVE: {
-            /* find the relevant path, then remove it from the KeyingSet */
-            KS_Path *ksp = BKE_keyingset_find_path(ks, id, nullptr, path, array_index, groupmode);
+    ID *id = nullptr;
+    char *path = nullptr;
+    int array_index = 0;
+    short flag = 0;
+    short groupmode = KSP_GROUP_KSNAME;
+
+    /* check if RNA-property described by this selected element is an animatable prop */
+    const TreeElementRNACommon *te_rna = tree_element_cast<TreeElementRNACommon>(te);
+    PointerRNA ptr = te_rna->getPointerRNA();
+    if (te_rna && te_rna->getPropertyRNA() &&
+        RNA_property_animateable(&ptr, te_rna->getPropertyRNA())) {
+      /* get id + path + index info from the selected element */
+      tree_element_to_path(te, tselem, &id, &path, &array_index, &flag, &groupmode);
+    }
 
-            if (ksp) {
-              /* free path's data */
-              BKE_keyingset_free_path(ks, ksp);
+    /* only if ID and path were set, should we perform any actions */
+    if (id && path) {
+      /* action depends on mode */
+      switch (mode) {
+        case KEYINGSET_EDITMODE_ADD: {
+          /* add a new path with the information obtained (only if valid) */
+          /* TODO: what do we do with group name?
+           * for now, we don't supply one, and just let this use the KeyingSet name */
+          BKE_keyingset_add_path(ks, id, nullptr, path, array_index, flag, groupmode);
+          ks->active_path = BLI_listbase_count(&ks->paths);
+          break;
+        }
+        case KEYINGSET_EDITMODE_REMOVE: {
+          /* find the relevant path, then remove it from the KeyingSet */
+          KS_Path *ksp = BKE_keyingset_find_path(ks, id, nullptr, path, array_index, groupmode);
 
-              ks->active_path = 0;
-            }
-            break;
+          if (ksp) {
+            /* free path's data */
+            BKE_keyingset_free_path(ks, ksp);
+
+            ks->active_path = 0;
           }
+          break;
         }
-
-        /* free path, since it had to be generated */
-        MEM_freeN(path);
       }
-    }
 
-    /* go over sub-tree */
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      do_outliner_keyingset_editop(space_outliner, ks, &te->subtree, mode);
+      /* free path, since it had to be generated */
+      MEM_freeN(path);
     }
-  }
+  });
 }
 
 /** \} */
@@ -2158,7 +2137,7 @@ static int outliner_keyingset_additems_exec(bContext *C, wmOperator *op)
   }
 
   /* recursively go into tree, adding selected items */
-  do_outliner_keyingset_editop(space_outliner, ks, &space_outliner->tree, KEYINGSET_EDITMODE_ADD);
+  do_outliner_keyingset_editop(space_outliner, ks, KEYINGSET_EDITMODE_ADD);
 
   /* send notifiers */
   WM_event_add_notifier(C, NC_SCENE | ND_KEYINGSET, nullptr);
@@ -2199,8 +2178,7 @@ static int outliner_keyingset_removeitems_exec(bContext *C, wmOperator *UNUSED(o
   }
 
   /* recursively go into tree, adding selected items */
-  do_outliner_keyingset_editop(
-      space_outliner, ks, &space_outliner->tree, KEYINGSET_EDITMODE_REMOVE);
+  do_outliner_keyingset_editop(space_outliner, ks, KEYINGSET_EDITMODE_REMOVE);
 
   /* send notifiers */
   WM_event_add_notifier(C, NC_SCENE | ND_KEYINGSET, nullptr);
diff --git a/source/blender/editors/space_outliner/outliner_intern.hh b/source/blender/editors/space_outliner/outliner_intern.hh
index 190b8f2fe36..a0dcb49aa43 100644
--- a/source/blender/editors/space_outliner/outliner_intern.hh
+++ b/source/blender/editors/space_outliner/outliner_intern.hh
@@ -408,8 +408,12 @@ int outliner_flag_is_any_test(ListBase *lb, short flag, int curlevel);
  * Set or unset \a flag for all outliner elements in \a lb and sub-trees.
  * \return if any flag was modified.
  */
-bool outliner_flag_set(ListBase *lb, short flag, short set);
-bool outliner_flag_flip(ListBase *lb, short flag);
+extern "C++" {
+bool outliner_flag_set(const SpaceOutliner &space_outliner, short flag, short set);
+bool outliner_flag_set(const ListBase &lb, short flag, short set);
+bool outliner_flag_flip(const SpaceOutliner &space_outliner, short flag);
+bool outliner_flag_flip(const ListBase &lb, short flag);
+}
 
 void item_rename_fn(struct bContext *C,
                     struct ReportList *reports,
@@ -451,7 +455,8 @@ void id_remap_fn(struct bContext *C,
 /**
  * To retrieve coordinates with redrawing the entire tree.
  */
-void outliner_set_coordinates(struct ARegion *region, struct SpaceOutliner *space_outliner);
+void outliner_set_coordinates(const struct ARegion *region,
+                              const struct SpaceOutliner *space_outliner);
 
 /**
  * Open or close a tree element, optionally toggling all children recursively.
diff --git a/source/blender/editors/space_outliner/outliner_select.cc b/source/blender/editors/space_outliner/outliner_select.cc
index f7c65bfcff2..bd6d3d89706 100644
--- a/source/blender/editors/space_outliner/outliner_select.cc
+++ b/source/blender/editors/space_outliner/outliner_select.cc
@@ -68,6 +68,7 @@
 #include "outliner_intern.hh"
 #include "tree/tree_display.hh"
 #include "tree/tree_element_seq.hh"
+#include "tree/tree_iterator.hh"
 
 using namespace blender::ed::outliner;
 
@@ -1457,7 +1458,7 @@ void outliner_item_select(bContext *C,
   /* Clear previous active when activating and clear selection when not extending selection */
   const short clear_flag = (activate ? TSE_ACTIVE : 0) | (extend ? 0 : TSE_SELECTED);
   if (clear_flag) {
-    outliner_flag_set(&space_outliner->tree, clear_flag, false);
+    outliner_flag_set(*space_outliner, clear_flag, false);
   }
 
   if (select_flag & OL_ITEM_SELECT) {
@@ -1531,7 +1532,7 @@ static void do_outliner_range_select(bContext *C,
   const bool active_selected = (tselem->flag & TSE_SELECTED);
 
   if (!extend) {
-    outliner_flag_set(&space_outliner->tree, TSE_SELECTED, false);
+    outliner_flag_set(*space_outliner, TSE_SELECTED, false);
   }
 
   /* Select active if under cursor */
@@ -1604,7 +1605,7 @@ static int outliner_item_do_activate_from_cursor(bContext *C,
 
   if (!(te = outliner_find_item_at_y(space_outliner, &space_outliner->tree, view_mval[1]))) {
     if (deselect_all) {
-      changed |= outliner_flag_set(&space_outliner->tree, TSE_SELECTED, false);
+      changed |= outliner_flag_set(*space_outliner, TSE_SELECTED, false);
     }
   }
   /* Don't allow toggle on scene collection */
@@ -1715,26 +1716,17 @@ void OUTLINER_OT_item_activate(wmOperatorType *ot)
 /** \name Box Select Operator
  * \{ */
 
-static void outliner_item_box_select(bContext *C,
-                                     SpaceOutliner *space_outliner,
-                                     Scene *scene,
-                                     rctf *rectf,
-                                     TreeElement *te,
-                                     bool select)
+static void outliner_box_select(bContext *C,
+                                SpaceOutliner *space_outliner,
+                                const rctf *rectf,
+                                const bool select)
 {
-  TreeStoreElem *tselem = TREESTORE(te);
-
-  if (te->ys <= rectf->ymax && te->ys + UI_UNIT_Y >= rectf->ymin) {
-    outliner_item_select(
-        C, space_outliner, te, (select ? OL_ITEM_SELECT : OL_ITEM_DESELECT) | OL_ITEM_EXTEND);
-  }
-
-  /* Look at its children. */
-  if (TSELEM_OPEN(tselem, space_outliner)) {
-    LISTBASE_FOREACH (TreeElement *, te_sub, &te->subtree) {
-      outliner_item_box_select(C, space_outliner, scene, rectf, te_sub, select);
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
+    if (te->ys <= rectf->ymax && te->ys + UI_UNIT_Y >= rectf->ymin) {
+      outliner_item_select(
+          C, space_outliner, te, (select ? OL_ITEM_SELECT : OL_ITEM_DESELECT) | OL_ITEM_EXTEND);
     }
-  }
+  });
 }
 
 static int outliner_box_select_exec(bContext *C, wmOperator *op)
@@ -1747,15 +1739,13 @@ static int outliner_box_select_exec(bContext *C, wmOperator *op)
   const eSelectOp sel_op = (eSelectOp)RNA_enum_get(op->ptr, "mode");
   const bool select = (sel_op != SEL_OP_SUB);
   if (SEL_OP_USE_PRE_DESELECT(sel_op)) {
-    outliner_flag_set(&space_outliner->tree, TSE_SELECTED, 0);
+    outliner_flag_set(*space_outliner, TSE_SELECTED, 0);
   }
 
   WM_operator_properties_border_to_rctf(op, &rectf);
   UI_view2d_region_to_view_rctf(&region->v2d, &rectf, &rectf);
 
-  LISTBASE_FOREACH (TreeElement *, te, &space_outliner->tree) {
-    outliner_item_box_select(C, space_outliner, scene, &rectf, te, select);
-  }
+  outliner_box_select(C, space_outliner, &rectf, select);
 
   DEG_id_tag_update(&scene->id, ID_RECALC_SELECT);
   WM_event_add_notifier(C, NC_SCENE | ND_OB_SELECT, scene);
diff --git a/source/blender/editors/space_outliner/outliner_tools.cc b/source/blender/editors/space_outliner/outliner_tools.cc
index 7321fd8fc7a..3b018d59881 100644
--- a/source/blender/editors/space_outliner/outliner_tools.cc
+++ b/source/blender/editors/space_outliner/outliner_tools.cc
@@ -84,6 +84,7 @@
 #include "outliner_intern.hh"
 #include "tree/tree_element_rna.hh"
 #include "tree/tree_element_seq.hh"
+#include "tree/tree_iterator.hh"
 
 static CLG_LogRef LOG = {"ed.outliner.tools"};
 
@@ -412,11 +413,10 @@ static void outliner_do_libdata_operation(bContext *C,
                                           ReportList *reports,
                                           Scene *scene,
                                           SpaceOutliner *space_outliner,
-                                          ListBase *lb,
                                           outliner_operation_fn operation_fn,
                                           void *user_data)
 {
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
     if (tselem->flag & TSE_SELECTED) {
       if (((tselem->type == TSE_SOME_ID) && (te->idcode != 0)) ||
@@ -425,11 +425,7 @@ static void outliner_do_libdata_operation(bContext *C,
         operation_fn(C, reports, scene, te, tsep, tselem, user_data);
       }
     }
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_do_libdata_operation(
-          C, reports, scene, space_outliner, &te->subtree, operation_fn, user_data);
-    }
-  }
+  });
 }
 
 /** \} */
@@ -1602,21 +1598,17 @@ static void outliner_do_data_operation(
     SpaceOutliner *space_outliner,
     int type,
     int event,
-    ListBase *lb,
     void (*operation_fn)(int, TreeElement *, TreeStoreElem *, void *),
     void *arg)
 {
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
     if (tselem->flag & TSE_SELECTED) {
       if (tselem->type == type) {
         operation_fn(event, te, tselem, arg);
       }
     }
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_do_data_operation(space_outliner, type, event, &te->subtree, operation_fn, arg);
-    }
-  }
+  });
 }
 
 static Base *outliner_batch_delete_hierarchy(
@@ -1780,8 +1772,7 @@ static int outliner_object_operation_exec(bContext *C, wmOperator *op)
       selection_changed = true;
       break;
     case OL_OP_REMAP:
-      outliner_do_libdata_operation(
-          C, op->reports, scene, space_outliner, &space_outliner->tree, id_remap_fn, nullptr);
+      outliner_do_libdata_operation(C, op->reports, scene, space_outliner, id_remap_fn, nullptr);
       /* No undo push here, operator does it itself (since it's a modal one, the op_undo_depth
        * trick does not work here). */
       break;
@@ -2192,13 +2183,8 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     case OUTLINER_IDOP_UNLINK: {
       /* unlink datablock from its parent */
       if (objectlevel) {
-        outliner_do_libdata_operation(C,
-                                      op->reports,
-                                      scene,
-                                      space_outliner,
-                                      &space_outliner->tree,
-                                      unlink_object_fn,
-                                      nullptr);
+        outliner_do_libdata_operation(
+            C, op->reports, scene, space_outliner, unlink_object_fn, nullptr);
 
         WM_event_add_notifier(C, NC_SCENE | ND_LAYER, nullptr);
         ED_undo_push(C, "Unlink Object");
@@ -2207,61 +2193,36 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
 
       switch (idlevel) {
         case ID_AC:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        unlink_action_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, unlink_action_fn, nullptr);
 
           WM_event_add_notifier(C, NC_ANIMATION | ND_NLA_ACTCHANGE, nullptr);
           ED_undo_push(C, "Unlink action");
           break;
         case ID_MA:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        unlink_material_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, unlink_material_fn, nullptr);
 
           WM_event_add_notifier(C, NC_OBJECT | ND_OB_SHADING, nullptr);
           ED_undo_push(C, "Unlink material");
           break;
         case ID_TE:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        unlink_texture_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, unlink_texture_fn, nullptr);
 
           WM_event_add_notifier(C, NC_OBJECT | ND_OB_SHADING, nullptr);
           ED_undo_push(C, "Unlink texture");
           break;
         case ID_WO:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        unlink_world_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, unlink_world_fn, nullptr);
 
           WM_event_add_notifier(C, NC_SCENE | ND_WORLD, nullptr);
           ED_undo_push(C, "Unlink world");
           break;
         case ID_GR:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        unlink_collection_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, unlink_collection_fn, nullptr);
 
           WM_event_add_notifier(C, NC_SCENE | ND_LAYER, nullptr);
           ED_undo_push(C, "Unlink Collection");
@@ -2274,20 +2235,14 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     }
     case OUTLINER_IDOP_LOCAL: {
       /* make local */
-      outliner_do_libdata_operation(
-          C, op->reports, scene, space_outliner, &space_outliner->tree, id_local_fn, nullptr);
+      outliner_do_libdata_operation(C, op->reports, scene, space_outliner, id_local_fn, nullptr);
       ED_undo_push(C, "Localized Data");
       break;
     }
     case OUTLINER_IDOP_OVERRIDE_LIBRARY_CREATE: {
       OutlinerLibOverrideData override_data{};
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_create_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_create_fn, &override_data);
       ED_undo_push(C, "Overridden Data");
       break;
     }
@@ -2298,16 +2253,10 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
                                     op->reports,
                                     scene,
                                     space_outliner,
-                                    &space_outliner->tree,
                                     id_override_library_create_hierarchy_pre_process_fn,
                                     &override_data);
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_create_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_create_fn, &override_data);
       id_override_library_create_hierarchy_post_process(C, &override_data);
 
       ED_undo_push(C, "Overridden Data Hierarchy");
@@ -2321,16 +2270,10 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
                                     op->reports,
                                     scene,
                                     space_outliner,
-                                    &space_outliner->tree,
                                     id_override_library_create_hierarchy_pre_process_fn,
                                     &override_data);
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_create_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_create_fn, &override_data);
       id_override_library_create_hierarchy_post_process(C, &override_data);
 
       ED_undo_push(C, "Overridden Data Hierarchy Fully Editable");
@@ -2341,7 +2284,6 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
                                     op->reports,
                                     scene,
                                     space_outliner,
-                                    &space_outliner->tree,
                                     id_override_library_toggle_flag_fn,
                                     POINTER_FROM_UINT(IDOVERRIDE_LIBRARY_FLAG_SYSTEM_DEFINED));
 
@@ -2350,39 +2292,24 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     }
     case OUTLINER_IDOP_OVERRIDE_LIBRARY_RESET: {
       OutlinerLibOverrideData override_data{};
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_reset_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_reset_fn, &override_data);
       ED_undo_push(C, "Reset Overridden Data");
       break;
     }
     case OUTLINER_IDOP_OVERRIDE_LIBRARY_RESET_HIERARCHY: {
       OutlinerLibOverrideData override_data{};
       override_data.do_hierarchy = true;
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_reset_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_reset_fn, &override_data);
       ED_undo_push(C, "Reset Overridden Data Hierarchy");
       break;
     }
     case OUTLINER_IDOP_OVERRIDE_LIBRARY_RESYNC_HIERARCHY: {
       OutlinerLibOverrideData override_data{};
       override_data.do_hierarchy = true;
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_resync_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_resync_fn, &override_data);
       ED_undo_push(C, "Resync Overridden Data Hierarchy");
       break;
     }
@@ -2390,35 +2317,20 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
       OutlinerLibOverrideData override_data{};
       override_data.do_hierarchy = true;
       override_data.do_resync_hierarchy_enforce = true;
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_resync_fn,
-                                    &override_data);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_resync_fn, &override_data);
       ED_undo_push(C, "Resync Overridden Data Hierarchy");
       break;
     }
     case OUTLINER_IDOP_OVERRIDE_LIBRARY_CLEAR_HIERARCHY: {
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_clear_hierarchy_fn,
-                                    nullptr);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_clear_hierarchy_fn, nullptr);
       ED_undo_push(C, "Clear Overridden Data Hierarchy");
       break;
     }
     case OUTLINER_IDOP_OVERRIDE_LIBRARY_CLEAR_SINGLE: {
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_override_library_clear_single_fn,
-                                    nullptr);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_override_library_clear_single_fn, nullptr);
       ED_undo_push(C, "Clear Overridden Data Hierarchy");
       break;
     }
@@ -2426,26 +2338,16 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
       /* make single user */
       switch (idlevel) {
         case ID_AC:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        singleuser_action_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, singleuser_action_fn, nullptr);
 
           WM_event_add_notifier(C, NC_ANIMATION | ND_NLA_ACTCHANGE, nullptr);
           ED_undo_push(C, "Single-User Action");
           break;
 
         case ID_WO:
-          outliner_do_libdata_operation(C,
-                                        op->reports,
-                                        scene,
-                                        space_outliner,
-                                        &space_outliner->tree,
-                                        singleuser_world_fn,
-                                        nullptr);
+          outliner_do_libdata_operation(
+              C, op->reports, scene, space_outliner, singleuser_world_fn, nullptr);
 
           WM_event_add_notifier(C, NC_SCENE | ND_WORLD, nullptr);
           ED_undo_push(C, "Single-User World");
@@ -2460,15 +2362,14 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     case OUTLINER_IDOP_DELETE: {
       if (idlevel > 0) {
         outliner_do_libdata_operation(
-            C, op->reports, scene, space_outliner, &space_outliner->tree, id_delete_fn, nullptr);
+            C, op->reports, scene, space_outliner, id_delete_fn, nullptr);
         ED_undo_push(C, "Delete");
       }
       break;
     }
     case OUTLINER_IDOP_REMAP: {
       if (idlevel > 0) {
-        outliner_do_libdata_operation(
-            C, op->reports, scene, space_outliner, &space_outliner->tree, id_remap_fn, nullptr);
+        outliner_do_libdata_operation(C, op->reports, scene, space_outliner, id_remap_fn, nullptr);
         /* No undo push here, operator does it itself (since it's a modal one, the op_undo_depth
          * trick does not work here). */
       }
@@ -2491,13 +2392,8 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     }
     case OUTLINER_IDOP_FAKE_ADD: {
       /* set fake user */
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_fake_user_set_fn,
-                                    nullptr);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_fake_user_set_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ID | NA_EDITED, nullptr);
       ED_undo_push(C, "Add Fake User");
@@ -2505,13 +2401,8 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     }
     case OUTLINER_IDOP_FAKE_CLEAR: {
       /* clear fake user */
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_fake_user_clear_fn,
-                                    nullptr);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_fake_user_clear_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ID | NA_EDITED, nullptr);
       ED_undo_push(C, "Clear Fake User");
@@ -2520,20 +2411,15 @@ static int outliner_id_operation_exec(bContext *C, wmOperator *op)
     case OUTLINER_IDOP_RENAME: {
       /* rename */
       outliner_do_libdata_operation(
-          C, op->reports, scene, space_outliner, &space_outliner->tree, item_rename_fn, nullptr);
+          C, op->reports, scene, space_outliner, item_rename_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ID | NA_EDITED, nullptr);
       ED_undo_push(C, "Rename");
       break;
     }
     case OUTLINER_IDOP_SELECT_LINKED:
-      outliner_do_libdata_operation(C,
-                                    op->reports,
-                                    scene,
-                                    space_outliner,
-                                    &space_outliner->tree,
-                                    id_select_linked_fn,
-                                    nullptr);
+      outliner_do_libdata_operation(
+          C, op->reports, scene, space_outliner, id_select_linked_fn, nullptr);
       ED_outliner_select_sync_from_all_tag(C);
       ED_undo_push(C, "Select");
       break;
@@ -2616,21 +2502,19 @@ static int outliner_lib_operation_exec(bContext *C, wmOperator *op)
   eOutlinerLibOpTypes event = (eOutlinerLibOpTypes)RNA_enum_get(op->ptr, "type");
   switch (event) {
     case OL_LIB_DELETE: {
-      outliner_do_libdata_operation(
-          C, op->reports, scene, space_outliner, &space_outliner->tree, id_delete_fn, nullptr);
+      outliner_do_libdata_operation(C, op->reports, scene, space_outliner, id_delete_fn, nullptr);
       ED_undo_push(C, "Delete Library");
       break;
     }
     case OL_LIB_RELOCATE: {
       outliner_do_libdata_operation(
-          C, op->reports, scene, space_outliner, &space_outliner->tree, lib_relocate_fn, nullptr);
+          C, op->reports, scene, space_outliner, lib_relocate_fn, nullptr);
       /* No undo push here, operator does it itself (since it's a modal one, the op_undo_depth
        * trick does not work here). */
       break;
     }
     case OL_LIB_RELOAD: {
-      outliner_do_libdata_operation(
-          C, op->reports, scene, space_outliner, &space_outliner->tree, lib_reload_fn, nullptr);
+      outliner_do_libdata_operation(C, op->reports, scene, space_outliner, lib_reload_fn, nullptr);
       /* No undo push here, operator does it itself (since it's a modal one, the op_undo_depth
        * trick does not work here). */
       break;
@@ -2673,11 +2557,10 @@ void OUTLINER_OT_lib_operation(wmOperatorType *ot)
 static void outliner_do_id_set_operation(
     SpaceOutliner *space_outliner,
     int type,
-    ListBase *lb,
     ID *newid,
     void (*operation_fn)(TreeElement *, TreeStoreElem *, TreeStoreElem *, ID *))
 {
-  LISTBASE_FOREACH (TreeElement *, te, lb) {
+  tree_iterator::all_open(*space_outliner, [&](TreeElement *te) {
     TreeStoreElem *tselem = TREESTORE(te);
     if (tselem->flag & TSE_SELECTED) {
       if (tselem->type == type) {
@@ -2685,10 +2568,7 @@ static void outliner_do_id_set_operation(
         operation_fn(te, tselem, tsep, newid);
       }
     }
-    if (TSELEM_OPEN(tselem, space_outliner)) {
-      outliner_do_id_set_operation(space_outliner, type, &te->subtree, newid, operation_fn);
-    }
-  }
+  });
 }
 
 static void actionset_id_fn(TreeElement *UNUSED(te),
@@ -2743,12 +2623,10 @@ static int outliner_action_set_exec(bContext *C, wmOperator *op)
 
   /* perform action if valid channel */
   if (datalevel == TSE_ANIM_DATA) {
-    outliner_do_id_set_operation(
-        space_outliner, datalevel, &space_outliner->tree, (ID *)act, actionset_id_fn);
+    outliner_do_id_set_operation(space_outliner, datalevel, (ID *)act, actionset_id_fn);
   }
   else if (idlevel == ID_AC) {
-    outliner_do_id_set_operation(
-        space_outliner, idlevel, &space_outliner->tree, (ID *)act, actionset_id_fn);
+    outliner_do_id_set_operation(space_outliner, idlevel, (ID *)act, actionset_id_fn);
   }
   else {
     return OPERATOR_CANCELLED;
@@ -2835,8 +2713,7 @@ static int outliner_animdata_operation_exec(bContext *C, wmOperator *op)
   switch (event) {
     case OUTLINER_ANIMOP_CLEAR_ADT:
       /* Remove Animation Data - this may remove the active action, in some cases... */
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, clear_animdata_fn, nullptr);
+      outliner_do_data_operation(space_outliner, datalevel, event, clear_animdata_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ANIMATION | ND_NLA_ACTCHANGE, nullptr);
       ED_undo_push(C, "Clear Animation Data");
@@ -2853,32 +2730,23 @@ static int outliner_animdata_operation_exec(bContext *C, wmOperator *op)
 
     case OUTLINER_ANIMOP_CLEAR_ACT:
       /* clear active action - using standard rules */
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, unlinkact_animdata_fn, nullptr);
+      outliner_do_data_operation(space_outliner, datalevel, event, unlinkact_animdata_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ANIMATION | ND_NLA_ACTCHANGE, nullptr);
       ED_undo_push(C, "Unlink action");
       break;
 
     case OUTLINER_ANIMOP_REFRESH_DRV:
-      outliner_do_data_operation(space_outliner,
-                                 datalevel,
-                                 event,
-                                 &space_outliner->tree,
-                                 refreshdrivers_animdata_fn,
-                                 nullptr);
+      outliner_do_data_operation(
+          space_outliner, datalevel, event, refreshdrivers_animdata_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ANIMATION | ND_ANIMCHAN, nullptr);
       // ED_undo_push(C, "Refresh Drivers"); /* No undo needed - shouldn't have any impact? */
       break;
 
     case OUTLINER_ANIMOP_CLEAR_DRV:
-      outliner_do_data_operation(space_outliner,
-                                 datalevel,
-                                 event,
-                                 &space_outliner->tree,
-                                 cleardrivers_animdata_fn,
-                                 nullptr);
+      outliner_do_data_operation(
+          space_outliner, datalevel, event, cleardrivers_animdata_fn, nullptr);
 
       WM_event_add_notifier(C, NC_ANIMATION | ND_ANIMCHAN, nullptr);
       ED_undo_push(C, "Clear Drivers");
@@ -2928,8 +2796,7 @@ static int outliner_constraint_operation_exec(bContext *C, wmOperator *op)
   SpaceOutliner *space_outliner = CTX_wm_space_outliner(C);
   eOutliner_PropConstraintOps event = (eOutliner_PropConstraintOps)RNA_enum_get(op->ptr, "type");
 
-  outliner_do_data_operation(
-      space_outliner, TSE_CONSTRAINT, event, &space_outliner->tree, constraint_fn, C);
+  outliner_do_data_operation(space_outliner, TSE_CONSTRAINT, event, constraint_fn, C);
 
   if (event == OL_CONSTRAINTOP_DELETE) {
     outliner_cleanup_tree(space_outliner);
@@ -2975,8 +2842,7 @@ static int outliner_modifier_operation_exec(bContext *C, wmOperator *op)
   SpaceOutliner *space_outliner = CTX_wm_space_outliner(C);
   eOutliner_PropModifierOps event = (eOutliner_PropModifierOps)RNA_enum_get(op->ptr, "type");
 
-  outliner_do_data_operation(
-      space_outliner, TSE_MODIFIER, event, &space_outliner->tree, modifier_fn, C);
+  outliner_do_data_operation(space_outliner, TSE_MODIFIER, event, modifier_fn, C);
 
   if (event == OL_MODIFIER_OP_DELETE) {
     outliner_cleanup_tree(space_outliner);
@@ -3019,24 +2885,21 @@ static int outliner_data_operation_exec(bContext *C, wmOperator *op)
   eOutliner_PropDataOps event = (eOutliner_PropDataOps)RNA_enum_get(op->ptr, "type");
   switch (datalevel) {
     case TSE_POSE_CHANNEL: {
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, pchan_fn, nullptr);
+      outliner_do_data_operation(space_outliner, datalevel, event, pchan_fn, nullptr);
       WM_event_add_notifier(C, NC_OBJECT | ND_POSE, nullptr);
       ED_undo_push(C, "PoseChannel operation");
 
       break;
     }
     case TSE_BONE: {
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, bone_fn, nullptr);
+      outliner_do_data_operation(space_outliner, datalevel, event, bone_fn, nullptr);
       WM_event_add_notifier(C, NC_OBJECT | ND_POSE, nullptr);
       ED_undo_push(C, "Bone operation");
 
       break;
     }
     case TSE_EBONE: {
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, ebone_fn, nullptr);
+      outliner_do_data_operation(space_outliner, datalevel, event, ebone_fn, nullptr);
       WM_event_add_notifier(C, NC_OBJECT | ND_POSE, nullptr);
       ED_undo_push(C, "EditBone operation");
 
@@ -3044,16 +2907,14 @@ static int outliner_data_operation_exec(bContext *C, wmOperator *op)
     }
     case TSE_SEQUENCE: {
       Scene *scene = CTX_data_scene(C);
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, sequence_fn, scene);
+      outliner_do_data_operation(space_outliner, datalevel, event, sequence_fn, scene);
       WM_event_add_notifier(C, NC_SCENE | ND_SEQUENCER | NA_SELECTED, scene);
       ED_undo_push(C, "Sequencer operation");
 
       break;
     }
     case TSE_GP_LAYER: {
-      outliner_do_data_operation(
-          space_outliner, datalevel, event, &space_outliner->tree, gpencil_layer_fn, nullptr);
+      outliner_do_data_operation(space_outliner, datalevel, event, gpencil_layer_fn, nullptr);
       WM_event_add_notifier(C, NC_GPENCIL | ND_DATA, nullptr);
       ED_undo_push(C, "Grease Pencil Layer operation");
 
@@ -3061,8 +2922,7 @@ static int outliner_data_operation_exec(bContext *C, wmOperator *op)
     }
     case TSE_RNA_STRUCT:
       if (event == OL_DOP_SELECT_LINKED) {
-        outliner_do_data_operation(
-            space_outliner, datalevel, event, &space_outliner->tree, data_select_linked_fn, C);
+        outliner_do_data_operation(space_outliner, datalevel, event, data_select_linked_fn, C);
       }
 
       break;
diff --git a/source/blender/editors/space_outliner/outliner_utils.cc b/source/blender/editors/space_outliner/outliner_utils.cc
index 4b947154864..0db612ce6db 100644
--- a/source/blender/editors/space_outliner/outliner_utils.cc
+++ b/source/blender/editors/space_outliner/outliner_utils.cc
@@ -27,6 +27,9 @@
 #include "UI_view2d.h"
 
 #include "outliner_intern.hh"
+#include "tree/tree_iterator.hh"
+
+using namespace blender::ed::outliner;
 
 /* -------------------------------------------------------------------- */
 /** \name Tree View Context
diff --git a/source/blender/editors/space_outliner/space_outliner.cc b/source/blender/editors/space_outliner/space_outliner.cc
index 97dc659155f..5bcd1edebc0 100644
--- a/source/blender/editors/space_outliner/space_outliner.cc
+++ b/source/blender/editors/space_outliner/space_outliner.cc
@@ -438,7 +438,7 @@ static void outliner_deactivate(struct ScrArea *area)
 {
   /* Remove hover highlights */
   SpaceOutliner *space_outliner = reinterpret_cast<SpaceOutliner *>(area->spacedata.first);
-  outliner_flag_set(&space_outliner->tree, TSE_HIGHLIGHTED_ANY, false);
+  outliner_flag_set(*space_outliner, TSE_HIGHLIGHTED_ANY, false);
   ED_region_tag_redraw_no_rebuild(BKE_area_find_region_type(area, RGN_TYPE_WINDOW));
 }