UI: Support UI list tooltips, defined via Python scripts

Makes it possible to create tooltips for UI list rows, which can be
filled in .py scripts, similar to how they can extend other menus. This
is used by the (to be committed) Pose Library add-on to display pose
operations (selecting bones of a pose, blending a pose, etc).

It's important that the Python scripts check if the UI list is the
correct one by checking the list ID.
For this to work, a new `bpy.context.ui_list` can be checked. For
example, the Pose Library add-on does the following check:
```
def is_pose_asset_view() -> bool:
  # Important: Must check context first, or the menu is added for every kind of list.
  list = getattr(context, "ui_list", None)
  if not list or list.bl_idname != "UI_UL_asset_view" or list.list_id != "pose_assets":
    return False
  if not context.asset_handle:
    return False
  return True
```

1 files changed, 15 insertions, 1 deletions

diff --git a/source/blender/editors/interface/interface_context_menu.c b/source/blender/editors/interface/interface_context_menu.c
index 2a7611eabb1..3049e2bd7b8 100644
--- a/source/blender/editors/interface/interface_context_menu.c
+++ b/source/blender/editors/interface/interface_context_menu.c
@@ -494,7 +494,7 @@ static void ui_but_menu_add_path_operators(uiLayout *layout, PointerRNA *ptr, Pr
   RNA_string_set(&props_ptr, "filepath", dir);
 }
 
-bool ui_popup_context_menu_for_button(bContext *C, uiBut *but)
+bool ui_popup_context_menu_for_button(bContext *C, uiBut *but, const wmEvent *event)
 {
   /* ui_but_is_interactive() may let some buttons through that should not get a context menu - it
    * doesn't make sense for them. */
@@ -1226,6 +1226,20 @@ bool ui_popup_context_menu_for_button(bContext *C, uiBut *but)
     }
   }
 
+  /* UI List item context menu. Scripts can add items to it, by default there's nothing shown. */
+  ARegion *region = CTX_wm_region(C);
+  const bool is_inside_listbox = ui_list_find_mouse_over(region, event) != NULL;
+  const bool is_inside_listrow = is_inside_listbox ?
+                                     ui_list_row_find_mouse_over(region, event->x, event->y) !=
+                                         NULL :
+                                     false;
+  if (is_inside_listrow) {
+    MenuType *mt = WM_menutype_find("UI_MT_list_item_context_menu", true);
+    if (mt) {
+      UI_menutype_draw(C, mt, uiLayoutColumn(layout, false));
+    }
+  }
+
   MenuType *mt = WM_menutype_find("WM_MT_button_context", true);
   if (mt) {
     UI_menutype_draw(C, mt, uiLayoutColumn(layout, false));