UI: add subpanel support.

In the Python API, any panel becomes a subpanel by setting bl_parent_id
to the name of the parent panel. These subpanels can contain advanced or
less commonly used settings.

15 files changed, 401 insertions, 188 deletions

diff --git a/source/blender/blenkernel/BKE_blender_version.h b/source/blender/blenkernel/BKE_blender_version.h
index 8befeff498e..a1212322983 100644
--- a/source/blender/blenkernel/BKE_blender_version.h
+++ b/source/blender/blenkernel/BKE_blender_version.h
@@ -28,7 +28,7 @@
  * and keep comment above the defines.
  * Use STRINGIFY() rather than defining with quotes */
 #define BLENDER_VERSION         280
-#define BLENDER_SUBVERSION      16
+#define BLENDER_SUBVERSION      17
 /* Several breakages with 270, e.g. constraint deg vs rad */
 #define BLENDER_MINVERSION      270
 #define BLENDER_MINSUBVERSION   6
diff --git a/source/blender/blenkernel/BKE_screen.h b/source/blender/blenkernel/BKE_screen.h
index f284e633c5e..0b95152ad8e 100644
--- a/source/blender/blenkernel/BKE_screen.h
+++ b/source/blender/blenkernel/BKE_screen.h
@@ -205,7 +205,8 @@ typedef struct PanelType {
 	char translation_context[BKE_ST_MAXNAME];
 	char context[BKE_ST_MAXNAME];             /* for buttons window */
 	char category[BKE_ST_MAXNAME];            /* for category tabs */
-	char owner_id[BKE_ST_MAXNAME];              /* for work-spaces to selectively show. */
+	char owner_id[BKE_ST_MAXNAME];            /* for work-spaces to selectively show. */
+	char parent_id[BKE_ST_MAXNAME];           /* parent idname for subpanels */
 	int space_type;
 	int region_type;
 
@@ -218,6 +219,10 @@ typedef struct PanelType {
 	/* draw entirely, view changes should be handled here */
 	void (*draw)(const struct bContext *C, struct Panel *pa);
 
+	/* sub panels */
+	struct PanelType *parent;
+	ListBase children;
+
 	/* RNA integration */
 	ExtensionRNA ext;
 } PanelType;
diff --git a/source/blender/blenkernel/intern/screen.c b/source/blender/blenkernel/intern/screen.c
index ece68884f5c..8cff10902ef 100644
--- a/source/blender/blenkernel/intern/screen.c
+++ b/source/blender/blenkernel/intern/screen.c
@@ -69,13 +69,19 @@ static void spacetype_free(SpaceType *st)
 	for (art = st->regiontypes.first; art; art = art->next) {
 		BLI_freelistN(&art->drawcalls);
 
-		for (pt = art->paneltypes.first; pt; pt = pt->next)
-			if (pt->ext.free)
+		for (pt = art->paneltypes.first; pt; pt = pt->next) {
+			if (pt->ext.free) {
 				pt->ext.free(pt->ext.data);
+			}
+
+			BLI_freelistN(&pt->children);
+		}
 
-		for (ht = art->headertypes.first; ht; ht = ht->next)
-			if (ht->ext.free)
+		for (ht = art->headertypes.first; ht; ht = ht->next) {
+			if (ht->ext.free) {
 				ht->ext.free(ht->ext.data);
+			}
+		}
 
 		BLI_freelistN(&art->paneltypes);
 		BLI_freelistN(&art->headertypes);
@@ -169,10 +175,35 @@ void BKE_spacedata_freelist(ListBase *lb)
 	BLI_freelistN(lb);
 }
 
+static void panel_list_copy(ListBase *newlb, const ListBase *lb)
+{
+	BLI_listbase_clear(newlb);
+	BLI_duplicatelist(newlb, lb);
+
+	/* copy panel pointers */
+	Panel *newpa = newlb->first;
+	Panel *pa = lb->first;
+	for (; newpa; newpa = newpa->next, pa = pa->next) {
+		newpa->activedata = NULL;
+
+		Panel *newpatab = newlb->first;
+		Panel *patab = lb->first;
+		while (newpatab) {
+			if (newpa->paneltab == patab) {
+				newpa->paneltab = newpatab;
+				break;
+			}
+			newpatab = newpatab->next;
+			patab = patab->next;
+		}
+
+		panel_list_copy(&newpa->children, &pa->children);
+	}
+}
+
 ARegion *BKE_area_region_copy(SpaceType *st, ARegion *ar)
 {
 	ARegion *newar = MEM_dupallocN(ar);
-	Panel *pa, *newpa, *patab;
 	
 	newar->prev = newar->next = NULL;
 	BLI_listbase_clear(&newar->handlers);
@@ -199,25 +230,10 @@ ARegion *BKE_area_region_copy(SpaceType *st, ARegion *ar)
 	if (ar->v2d.tab_offset)
 		newar->v2d.tab_offset = MEM_dupallocN(ar->v2d.tab_offset);
 	
-	BLI_listbase_clear(&newar->panels);
-	BLI_duplicatelist(&newar->panels, &ar->panels);
+	panel_list_copy(&newar->panels, &ar->panels);
 
 	BLI_listbase_clear(&newar->ui_previews);
 	BLI_duplicatelist(&newar->ui_previews, &ar->ui_previews);
-
-	/* copy panel pointers */
-	for (newpa = newar->panels.first; newpa; newpa = newpa->next) {
-		patab = newar->panels.first;
-		pa = ar->panels.first;
-		while (patab) {
-			if (newpa->paneltab == pa) {
-				newpa->paneltab = patab;
-				break;
-			}
-			patab = patab->next;
-			pa = pa->next;
-		}
-	}
 	
 	return newar;
 }
@@ -329,6 +345,19 @@ void BKE_region_callback_free_manipulatormap_set(void (*callback)(struct wmManip
 	region_free_manipulatormap_callback = callback;
 }
 
+static void panel_list_free(ListBase *lb)
+{
+	Panel *pa, *pa_next;
+	for (pa = lb->first; pa; pa = pa_next) {
+		pa_next = pa->next;
+		if (pa->activedata) {
+			MEM_freeN(pa->activedata);
+		}
+		panel_list_free(&pa->children);
+		MEM_freeN(pa);
+	}
+}
+
 /* not region itself */
 void BKE_area_region_free(SpaceType *st, ARegion *ar)
 {
@@ -351,16 +380,7 @@ void BKE_area_region_free(SpaceType *st, ARegion *ar)
 		ar->v2d.tab_offset = NULL;
 	}
 
-	if (!BLI_listbase_is_empty(&ar->panels)) {
-		Panel *pa, *pa_next;
-		for (pa = ar->panels.first; pa; pa = pa_next) {
-			pa_next = pa->next;
-			if (pa->activedata) {
-				MEM_freeN(pa->activedata);
-			}
-			MEM_freeN(pa);
-		}
-	}
+	panel_list_free(&ar->panels);
 
 	for (uilst = ar->ui_lists.first; uilst; uilst = uilst->next) {
 		if (uilst->dyn_data) {
diff --git a/source/blender/blenloader/intern/readfile.c b/source/blender/blenloader/intern/readfile.c
index d2542e977e3..b0a1789eb7e 100644
--- a/source/blender/blenloader/intern/readfile.c
+++ b/source/blender/blenloader/intern/readfile.c
@@ -6396,19 +6396,24 @@ static void direct_link_gpencil(FileData *fd, bGPdata *gpd)
 
 /* *********** READ AREA **************** */
 
-static void direct_link_region(FileData *fd, ARegion *ar, int spacetype)
+static void direct_link_panel_list(FileData *fd, ListBase *lb)
 {
-	Panel *pa;
-	uiList *ui_list;
-
-	link_list(fd, &ar->panels);
+	link_list(fd, lb);
 
-	for (pa = ar->panels.first; pa; pa = pa->next) {
+	for (Panel *pa = lb->first; pa; pa = pa->next) {
 		pa->paneltab = newdataadr(fd, pa->paneltab);
 		pa->runtime_flag = 0;
 		pa->activedata = NULL;
 		pa->type = NULL;
+		direct_link_panel_list(fd, &pa->children);
 	}
+}
+
+static void direct_link_region(FileData *fd, ARegion *ar, int spacetype)
+{
+	uiList *ui_list;
+
+	direct_link_panel_list(fd, &ar->panels);
 
 	link_list(fd, &ar->panels_category_active);
 
diff --git a/source/blender/blenloader/intern/writefile.c b/source/blender/blenloader/intern/writefile.c
index de1699e24b7..fb7b3e47153 100644
--- a/source/blender/blenloader/intern/writefile.c
+++ b/source/blender/blenloader/intern/writefile.c
@@ -2762,14 +2762,19 @@ static void write_soops(WriteData *wd, SpaceOops *so)
 	}
 }
 
+static void write_panel_list(WriteData *wd, ListBase *lb)
+{
+	for (Panel *pa = lb->first; pa; pa = pa->next) {
+		writestruct(wd, DATA, Panel, 1, pa);
+		write_panel_list(wd, &pa->children);
+	}
+}
+
 static void write_area_regions(WriteData *wd, ScrArea *area)
 {
 	for (ARegion *region = area->regionbase.first; region; region = region->next) {
 		write_region(wd, region, area->spacetype);
-
-		for (Panel *pa = region->panels.first; pa; pa = pa->next) {
-			writestruct(wd, DATA, Panel, 1, pa);
-		}
+		write_panel_list(wd, &region->panels);
 
 		for (PanelCategoryStack *pc_act = region->panels_category_active.first; pc_act; pc_act = pc_act->next) {
 			writestruct(wd, DATA, PanelCategoryStack, 1, pc_act);
diff --git a/source/blender/editors/include/UI_interface.h b/source/blender/editors/include/UI_interface.h
index 5779d6410a8..dc9de9f275f 100644
--- a/source/blender/editors/include/UI_interface.h
+++ b/source/blender/editors/include/UI_interface.h
@@ -491,8 +491,6 @@ void UI_blocklist_update_window_matrix(const struct bContext *C, const struct Li
 void UI_blocklist_draw(const struct bContext *C, const struct ListBase *lb);
 void UI_block_update_from_old(const struct bContext *C, struct uiBlock *block);
 
-uiBlock *UI_block_find_in_region(const char *name, struct ARegion *ar);
-
 void UI_block_emboss_set(uiBlock *block, char dt);
 
 void UI_block_free(const struct bContext *C, uiBlock *block);
@@ -824,9 +822,10 @@ void UI_panels_begin(const struct bContext *C, struct ARegion *ar);
 void UI_panels_end(const struct bContext *C, struct ARegion *ar, int *x, int *y);
 void UI_panels_draw(const struct bContext *C, struct ARegion *ar);
 
-struct Panel *UI_panel_find_by_type(struct ARegion *ar, struct PanelType *pt);
-struct Panel *UI_panel_begin(struct ScrArea *sa, struct ARegion *ar, uiBlock *block,
-                             struct PanelType *pt, struct Panel *pa, bool *r_open);
+struct Panel *UI_panel_find_by_type(struct ListBase *lb, struct PanelType *pt);
+struct Panel *UI_panel_begin(struct ScrArea *sa, struct ARegion *ar, struct ListBase *lb,
+                             uiBlock *block, struct PanelType *pt, struct Panel *pa,
+                             bool *r_open);
 void UI_panel_end(uiBlock *block, int width, int height);
 void UI_panels_scale(struct ARegion *ar, float new_width);
 
diff --git a/source/blender/editors/include/UI_resources.h b/source/blender/editors/include/UI_resources.h
index b89a57b02d1..f47daf1a963 100644
--- a/source/blender/editors/include/UI_resources.h
+++ b/source/blender/editors/include/UI_resources.h
@@ -74,6 +74,7 @@ typedef enum ThemeColorID {
 	/* panels */
 	TH_PANEL_HEADER,
 	TH_PANEL_BACK,
+	TH_PANEL_SUB_BACK,
 	TH_PANEL_SHOW_HEADER,
 	TH_PANEL_SHOW_BACK,
 	
diff --git a/source/blender/editors/interface/interface.c b/source/blender/editors/interface/interface.c
index a9995001659..6cb667cb9a8 100644
--- a/source/blender/editors/interface/interface.c
+++ b/source/blender/editors/interface/interface.c
@@ -2814,11 +2814,6 @@ uiBlock *UI_block_begin(const bContext *C, ARegion *region, const char *name, sh
 	return block;
 }
 
-uiBlock *UI_block_find_in_region(const char *name, ARegion *ar)
-{
-	return BLI_findstring(&ar->uiblocks, name, offsetof(uiBlock, name));
-}
-
 void UI_block_emboss_set(uiBlock *block, char dt)
 {
 	block->dt = dt;
diff --git a/source/blender/editors/interface/interface_panel.c b/source/blender/editors/interface/interface_panel.c
index 5711f76e3e9..3847aa72519 100644
--- a/source/blender/editors/interface/interface_panel.c
+++ b/source/blender/editors/interface/interface_panel.c
@@ -110,6 +110,7 @@ typedef struct uiHandlePanelData {
 	int startsizex, startsizey;
 } uiHandlePanelData;
 
+static int get_panel_real_size_y(Panel *pa);
 static void panel_activate_state(const bContext *C, Panel *pa, uiHandlePanelState state);
 
 /*********************** space specific code ************************/
@@ -133,45 +134,80 @@ static int panel_aligned(ScrArea *sa, ARegion *ar)
 	return 0;
 }
 
-static int panels_re_align(ScrArea *sa, ARegion *ar, Panel **r_pa)
+static bool panel_active_animation_changed(ListBase *lb, Panel **pa_animation, bool *no_animation)
 {
-	Panel *pa;
-	int active = 0;
+	for (Panel *pa = lb->first; pa; pa = pa->next) {
+		/* Detect panel active flag changes. */
+		if (!(pa->type && pa->type->parent)) {
+			if ((pa->runtime_flag & PNL_WAS_ACTIVE) && !(pa->runtime_flag & PNL_ACTIVE)) {
+				return true;
+			}
+			if (!(pa->runtime_flag & PNL_WAS_ACTIVE) && (pa->runtime_flag & PNL_ACTIVE)) {
+				return true;
+			}
+		}
 
-	*r_pa = NULL;
+		if ((pa->runtime_flag & PNL_ACTIVE) && !(pa->flag & PNL_CLOSED)) {
+			if (panel_active_animation_changed(&pa->children, pa_animation, no_animation)) {
+				return true;
+			}
+		}
+
+		/* Detect animation. */
+		if (pa->activedata) {
+			uiHandlePanelData *data = pa->activedata;
+			if (data->state == PANEL_STATE_ANIMATION) {
+				*pa_animation = pa;
+			}
+			else {
+				/* Don't animate while handling other interaction. */
+				*no_animation = true;
+			}
+		}
+		if ((pa->runtime_flag & PNL_ANIM_ALIGN) && !(*pa_animation)) {
+			*pa_animation = pa;
+		}
+	}
+
+	return false;
+}
+
+static bool panels_need_realign(ScrArea *sa, ARegion *ar, Panel **pa_animate)
+{
+	*pa_animate = NULL;
 
 	if (sa->spacetype == SPACE_BUTS && ar->regiontype == RGN_TYPE_WINDOW) {
 		SpaceButs *sbuts = sa->spacedata.first;
 
-		if (sbuts->align)
-			if (sbuts->re_align || sbuts->mainbo != sbuts->mainb)
-				return 1;
+		if (sbuts->align) {
+			if (sbuts->re_align || sbuts->mainbo != sbuts->mainb) {
+				return true;
+			}
+		}
+	}
+	else if (sa->spacetype == SPACE_IMAGE && ar->regiontype == RGN_TYPE_PREVIEW) {
+		return true;
+	}
+	else if (sa->spacetype == SPACE_FILE && ar->regiontype == RGN_TYPE_CHANNELS) {
+		return true;
 	}
-	else if (sa->spacetype == SPACE_IMAGE && ar->regiontype == RGN_TYPE_PREVIEW)
-		return 1;
-	else if (sa->spacetype == SPACE_FILE && ar->regiontype == RGN_TYPE_CHANNELS)
-		return 1;
 
-	/* in case panel is added or disappears */
-	for (pa = ar->panels.first; pa; pa = pa->next) {
-		if ((pa->runtime_flag & PNL_WAS_ACTIVE) && !(pa->runtime_flag & PNL_ACTIVE))
-			return 1;
-		if (!(pa->runtime_flag & PNL_WAS_ACTIVE) && (pa->runtime_flag & PNL_ACTIVE))
-			return 1;
-		if (pa->activedata)
-			active = 1;
+	/* Detect if a panel was added or removed. */
+	Panel *pa_animation = NULL;
+	bool no_animation = false;
+	if (panel_active_animation_changed(&ar->panels, &pa_animation, &no_animation)) {
+		return true;
 	}
 
-	/* in case we need to do an animation (size changes) */
-	for (pa = ar->panels.first; pa; pa = pa->next) {
-		if (pa->runtime_flag & PNL_ANIM_ALIGN) {
-			if (!active)
-				*r_pa = pa;
-			return 1;
+	/* Detect panel marked for animation, if we're not already animating. */
+	if (pa_animation) {
+		if (!no_animation) {
+			*pa_animate = pa_animation;
 		}
+		return true;
 	}
 
-	return 0;
+	return false;
 }
 
 /****************************** panels ******************************/
@@ -215,14 +251,14 @@ static void ui_panel_copy_offset(Panel *pa, Panel *papar)
  */
 /* #define UI_USE_PANELTAB */
 
-Panel *UI_panel_find_by_type(ARegion *ar, PanelType *pt)
+Panel *UI_panel_find_by_type(ListBase *lb, PanelType *pt)
 {
 	Panel *pa;
 	const char *idname = pt->idname;
 
 #ifdef UI_USE_PANELTAB
 	const char *tabname = pt->idname;
-	for (pa = ar->panels.first; pa; pa = pa->next) {
+	for (pa = lb->first; pa; pa = pa->next) {
 		if (STREQLEN(pa->panelname, idname, sizeof(pa->panelname))) {
 			if (STREQLEN(pa->tabname, tabname, sizeof(pa->tabname))) {
 				return pa;
@@ -230,7 +266,7 @@ Panel *UI_panel_find_by_type(ARegion *ar, PanelType *pt)
 		}
 	}
 #else
-	for (pa = ar->panels.first; pa; pa = pa->next) {
+	for (pa = lb->first; pa; pa = pa->next) {
 		if (STREQLEN(pa->panelname, idname, sizeof(pa->panelname))) {
 			return pa;
 		}
@@ -243,7 +279,7 @@ Panel *UI_panel_find_by_type(ARegion *ar, PanelType *pt)
 /**
  * \note \a pa should be return value from #UI_panel_find_by_type and can be NULL.
  */
-Panel *UI_panel_begin(ScrArea *sa, ARegion *ar, uiBlock *block, PanelType *pt, Panel *pa, bool *r_open)
+Panel *UI_panel_begin(ScrArea *sa, ARegion *ar, ListBase *lb, uiBlock *block, PanelType *pt, Panel *pa, bool *r_open)
 {
 	Panel *palast, *panext;
 	const char *drawname = CTX_IFACE_(pt->translation_context, pt->label);
@@ -275,9 +311,11 @@ Panel *UI_panel_begin(ScrArea *sa, ARegion *ar, uiBlock *block, PanelType *pt, P
 		pa->ofsy = 0;
 		pa->sizex = 0;
 		pa->sizey = 0;
+		pa->blocksizex = 0;
+		pa->blocksizey = 0;
 		pa->runtime_flag |= PNL_NEW_ADDED;
 
-		BLI_addtail(&ar->panels, pa);
+		BLI_addtail(lb, pa);
 
 #ifdef UI_USE_PANELTAB
 		BLI_strncpy(pa->tabname, tabname, sizeof(pa->tabname));
@@ -285,7 +323,7 @@ Panel *UI_panel_begin(ScrArea *sa, ARegion *ar, uiBlock *block, PanelType *pt, P
 		/* make new Panel tabbed? */
 		if (hookname) {
 			Panel *patab;
-			for (patab = ar->panels.first; patab; patab = patab->next) {
+			for (patab = lb->first; patab; patab = patab->next) {
 				if ((patab->runtime_flag & PNL_ACTIVE) && patab->paneltab == NULL) {
 					if (STREQLEN(hookname, patab->panelname, sizeof(patab->panelname))) {
 						if (STREQLEN(tabname, patab->tabname, sizeof(patab->tabname))) {
@@ -308,6 +346,8 @@ Panel *UI_panel_begin(ScrArea *sa, ARegion *ar, uiBlock *block, PanelType *pt, P
 		/* Force update of panels' positions! */
 		pa->sizex = 0;
 		pa->sizey = 0;
+		pa->blocksizex = 0;
+		pa->blocksizey = 0;
 	}
 
 	BLI_strncpy(pa->drawname, drawname, sizeof(pa->drawname));
@@ -315,14 +355,18 @@ Panel *UI_panel_begin(ScrArea *sa, ARegion *ar, uiBlock *block, PanelType *pt, P
 	/* if a new panel is added, we insert it right after the panel
 	 * that was last added. this way new panels are inserted in the
 	 * right place between versions */
-	for (palast = ar->panels.first; palast; palast = palast->next)
-		if (palast->runtime_flag & PNL_LAST_ADDED)
+	for (palast = lb->first; palast; palast = palast->next) {
+		if (palast->runtime_flag & PNL_LAST_ADDED) {
+			BLI_remlink(lb, pa);
+			BLI_insertlinkafter(lb, palast, pa);
 			break;
+		}
+	}
 
 	if (newpanel) {
 		pa->sortorder = (palast) ? palast->sortorder + 1 : 0;
 
-		for (panext = ar->panels.first; panext; panext = panext->next)
+		for (panext = lb->first; panext; panext = panext->next)
 			if (panext != pa && panext->sortorder >= pa->sortorder)
 				panext->sortorder++;
 	}
@@ -348,6 +392,19 @@ void UI_panel_end(uiBlock *block, int width, int height)
 {
 	Panel *pa = block->panel;
 
+	/* Set panel size excluding children. */
+	pa->blocksizex = width;
+	pa->blocksizey = height;
+
+	/* Compute total panel size including children. */
+	for (Panel *pachild = pa->children.first; pachild; pachild = pachild->next) {
+		if (pachild->runtime_flag & PNL_ACTIVE) {
+			width = max_ii(width, pachild->sizex);
+			height += get_panel_real_size_y(pachild);
+		}
+	}
+
+	/* Update total panel size. */
 	if (pa->runtime_flag & PNL_NEW_ADDED) {
 		pa->runtime_flag &= ~PNL_NEW_ADDED;
 		pa->sizex = width;
@@ -372,15 +429,13 @@ void UI_panel_end(uiBlock *block, int width, int height)
 static void ui_offset_panel_block(uiBlock *block)
 {
 	uiStyle *style = UI_style_get_dpi();
-	uiBut *but;
-	int ofsy;
 
 	/* compute bounds and offset */
 	ui_block_bounds_calc(block);
 
-	ofsy = block->panel->sizey - style->panelspace;
+	int ofsy = block->panel->sizey - style->panelspace;
 
-	for (but = block->buttons.first; but; but = but->next) {
+	for (uiBut *but = block->buttons.first; but; but = but->next) {
 		but->rect.ymin += ofsy;
 		but->rect.ymax += ofsy;
 	}
@@ -600,6 +655,8 @@ void ui_draw_aligned_panel(uiStyle *style, uiBlock *block, const rcti *rect, con
 	float color[4];
 	const bool is_closed_x = (panel->flag & PNL_CLOSEDX) ? true : false;
 	const bool is_closed_y = (panel->flag & PNL_CLOSEDY) ? true : false;
+	const bool is_subpanel = (panel->type && panel->type->parent);
+	const bool show_drag = !is_subpanel;
 
 	if (panel->paneltab) return;
 	if (panel->type && (panel->type->flag & PNL_NO_HEADER)) return;
@@ -613,7 +670,7 @@ void ui_draw_aligned_panel(uiStyle *style, uiBlock *block, const rcti *rect, con
 	unsigned int pos = GWN_vertformat_attr_add(immVertexFormat(), "pos", GWN_COMP_F32, 2, GWN_FETCH_FLOAT);
 	immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
 
-	{
+	if (!is_subpanel) {
 		float minx = rect->xmin;
 		float maxx = is_closed_x ? (minx + PNL_HEADER / block->aspect) : rect->xmax;
 		float y = headrect.ymax;
@@ -681,25 +738,28 @@ void ui_draw_aligned_panel(uiStyle *style, uiBlock *block, const rcti *rect, con
 
 	/* horizontal title */
 	if (is_closed_x == false) {
-		unsigned int col;
 		ui_draw_aligned_panel_header(style, block, &headrect, 'h');
-		Gwn_VertFormat *format = immVertexFormat();
-		pos = GWN_vertformat_attr_add(format, "pos", GWN_COMP_F32, 2, GWN_FETCH_FLOAT);
-		col = GWN_vertformat_attr_add(format, "color", GWN_COMP_F32, 4, GWN_FETCH_FLOAT);
-
-		/* itemrect smaller */
-		itemrect.xmax = headrect.xmax - 5.0f / block->aspect;
-		itemrect.xmin = itemrect.xmax - BLI_rcti_size_y(&headrect);
-		itemrect.ymin = headrect.ymin;
-		itemrect.ymax = headrect.ymax;
-
-		BLI_rctf_scale(&itemrect, 0.7f);
-		immBindBuiltinProgram(GPU_SHADER_2D_FLAT_COLOR);
-		ui_draw_panel_dragwidget(pos, col, &itemrect);
-		immUnbindProgram();
 
-		/* Restore format for the following draws. */
-		pos = GWN_vertformat_attr_add(immVertexFormat(), "pos", GWN_COMP_F32, 2, GWN_FETCH_FLOAT);
+		if (show_drag) {
+			unsigned int col;
+			Gwn_VertFormat *format = immVertexFormat();
+			pos = GWN_vertformat_attr_add(format, "pos", GWN_COMP_F32, 2, GWN_FETCH_FLOAT);
+			col = GWN_vertformat_attr_add(format, "color", GWN_COMP_F32, 4, GWN_FETCH_FLOAT);
+
+			/* itemrect smaller */
+			itemrect.xmax = headrect.xmax - 5.0f / block->aspect;
+			itemrect.xmin = itemrect.xmax - BLI_rcti_size_y(&headrect);
+			itemrect.ymin = headrect.ymin;
+			itemrect.ymax = headrect.ymax;
+
+			BLI_rctf_scale(&itemrect, 0.7f);
+			immBindBuiltinProgram(GPU_SHADER_2D_FLAT_COLOR);
+			ui_draw_panel_dragwidget(pos, col, &itemrect);
+			immUnbindProgram();
+
+			/* Restore format for the following draws. */
+			pos = GWN_vertformat_attr_add(immVertexFormat(), "pos", GWN_COMP_F32, 2, GWN_FETCH_FLOAT);
+		}
 	}
 
 	/* if the panel is minimized vertically:
@@ -727,8 +787,12 @@ void ui_draw_aligned_panel(uiStyle *style, uiBlock *block, const rcti *rect, con
 		immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
 
 		/* panel backdrop */
-		if (UI_GetThemeValue(TH_PANEL_SHOW_BACK)) {
-			/* draw with background color */
+		if (is_subpanel) {
+			glEnable(GL_BLEND);
+			immUniformThemeColor(TH_PANEL_SUB_BACK);
+			immRectf(pos, rect->xmin, rect->ymin, rect->xmax, rect->ymax);
+		}
+		else if (UI_GetThemeValue(TH_PANEL_SHOW_BACK)) {
 			glEnable(GL_BLEND);
 			immUniformThemeColor(TH_PANEL_BACK);
 			immRectf(pos, rect->xmin, rect->ymin, rect->xmax, rect->ymax);
@@ -786,6 +850,16 @@ static int get_panel_size_y(Panel *pa)
 	return PNL_HEADER + pa->sizey;
 }
 
+static int get_panel_real_size_y(Panel *pa)
+{
+	int sizey = (pa->flag & PNL_CLOSED) ? 0 : pa->sizey;
+
+	if (pa->type && (pa->type->flag & PNL_NO_HEADER))
+		return sizey;
+
+	return PNL_HEADER + sizey;
+}
+
 /* this function is needed because uiBlock and Panel itself don't
  * change sizey or location when closed */
 static int get_panel_real_ofsy(Panel *pa)
@@ -858,6 +932,24 @@ static int compare_panel(const void *a1, const void *a2)
 	return 0;
 }
 
+static void align_sub_panels(Panel *pa)
+{
+	/* Position sub panels. */
+	int ofsy = get_panel_real_ofsy(pa) + pa->sizey - pa->blocksizey;
+
+	for (Panel *pachild = pa->children.first; pachild; pachild = pachild->next) {
+		if (pachild->runtime_flag & PNL_ACTIVE) {
+			pachild->ofsx = pa->ofsx;
+			pachild->ofsy = ofsy - get_panel_size_y(pachild);
+			ofsy -= get_panel_real_size_y(pachild);
+
+			if (pachild->children.first) {
+				align_sub_panels(pachild);
+			}
+		}
+	}
+}
+
 /* this doesnt draw */
 /* returns 1 when it did something */
 static bool uiAlignPanelStep(ScrArea *sa, ARegion *ar, const float fac, const bool drag)
@@ -949,10 +1041,17 @@ static bool uiAlignPanelStep(ScrArea *sa, ARegion *ar, const float fac, const bo
 		}
 	}
 
-	/* copy locations to tabs */
-	for (pa = ar->panels.first; pa; pa = pa->next)
-		if (pa->paneltab && (pa->runtime_flag & PNL_ACTIVE))
-			ui_panel_copy_offset(pa, pa->paneltab);
+	/* set locations for tabbed and sub panels */
+	for (pa = ar->panels.first; pa; pa = pa->next) {
+		if (pa->runtime_flag & PNL_ACTIVE) {
+			if (pa->paneltab) {
+				ui_panel_copy_offset(pa, pa->paneltab);
+			}
+			if (pa->children.first) {
+				align_sub_panels(pa);
+			}
+		}
+	}
 
 	/* free panelsort array */
 	for (ps = panelsort, a = 0; a < tot; a++, ps++) {
@@ -1022,20 +1121,27 @@ static void ui_do_animate(const bContext *C, Panel *panel)
 	}
 }
 
-void UI_panels_begin(const bContext *UNUSED(C), ARegion *ar)
+static void panel_list_clear_active(ListBase *lb)
 {
-	Panel *pa;
-
 	/* set all panels as inactive, so that at the end we know
 	 * which ones were used */
-	for (pa = ar->panels.first; pa; pa = pa->next) {
-		if (pa->runtime_flag & PNL_ACTIVE)
+	for (Panel *pa = lb->first; pa; pa = pa->next) {
+		if (pa->runtime_flag & PNL_ACTIVE) {
 			pa->runtime_flag = PNL_WAS_ACTIVE;
-		else
+		}
+		else {
 			pa->runtime_flag = 0;
+		}
+
+		panel_list_clear_active(&pa->children);
 	}
 }
 
+void UI_panels_begin(const bContext *UNUSED(C), ARegion *ar)
+{
+	panel_list_clear_active(&ar->panels);
+}
+
 /* only draws blocks with panels */
 void UI_panels_end(const bContext *C, ARegion *ar, int *x, int *y)
 {
@@ -1074,8 +1180,7 @@ void UI_panels_end(const bContext *C, ARegion *ar, int *x, int *y)
 	}
 
 	/* re-align, possibly with animation */
-	if (panels_re_align(sa, ar, &pa)) {
-		/* XXX code never gets here... PNL_ANIM_ALIGN flag is never set */
+	if (panels_need_realign(sa, ar, &pa)) {
 		if (pa)
 			panel_activate_state(C, pa, PANEL_STATE_ANIMATION);
 		else
@@ -1102,14 +1207,16 @@ void UI_panels_draw(const bContext *C, ARegion *ar)
 
 	UI_ThemeClearColor(TH_BACK);
 
-	/* draw panels, selected on top */
-	for (block = ar->uiblocks.first; block; block = block->next) {
+	/* Draw panels, selected on top. Also in reverse order, because
+	 * UI blocks are added in reverse order and we need child panels
+	 * to draw on top. */
+	for (block = ar->uiblocks.last; block; block = block->prev) {
 		if (block->active && block->panel && !(block->panel->flag & PNL_SELECT)) {
 			UI_block_draw(C, block);
 		}
 	}
 
-	for (block = ar->uiblocks.first; block; block = block->next) {
+	for (block = ar->uiblocks.last; block; block = block->prev) {
 		if (block->active && block->panel && (block->panel->flag & PNL_SELECT)) {
 			UI_block_draw(C, block);
 		}
@@ -1371,6 +1478,8 @@ static void ui_handle_panel_header(const bContext *C, uiBlock *block, int mx, in
 #else
 	const bool show_pin = UI_panel_category_is_visible(ar);
 #endif
+	const bool is_subpanel = (block->panel->type && block->panel->type->parent);
+	const bool show_drag = !is_subpanel;
 
 	int align = panel_aligned(sa, ar), button = 0;
 
@@ -1465,7 +1574,7 @@ static void ui_handle_panel_header(const bContext *C, uiBlock *block, int mx, in
 		else
 			ED_region_tag_redraw(ar);
 	}
-	else if (BLI_rctf_isect_x(&rect_drag, mx)) {
+	else if (show_drag && BLI_rctf_isect_x(&rect_drag, mx)) {
 		panel_activate_state(C, block->panel, PANEL_STATE_DRAG);
 	}
 	else if (show_pin && BLI_rctf_isect_x(&rect_pin, mx)) {
diff --git a/source/blender/editors/interface/resources.c b/source/blender/editors/interface/resources.c
index 8e5bf8f0323..1bfb050563d 100644
--- a/source/blender/editors/interface/resources.c
+++ b/source/blender/editors/interface/resources.c
@@ -254,6 +254,8 @@ const unsigned char *UI_ThemeGetColorPtr(bTheme *btheme, int spacetype, int colo
 					cp = ts->panelcolors.header; break;
 				case TH_PANEL_BACK:
 					cp = ts->panelcolors.back; break;
+				case TH_PANEL_SUB_BACK:
+					cp = ts->panelcolors.sub_back; break;
 				case TH_PANEL_SHOW_HEADER:
 					cp = &setting;
 					setting = ts->panelcolors.show_header;
@@ -821,6 +823,7 @@ static void ui_theme_init_new_do(ThemeSpace *ts)
 	ts->panelcolors.show_header = false;
 	rgba_char_args_set(ts->panelcolors.back,   114, 114, 114, 128);
 	rgba_char_args_set(ts->panelcolors.header, 0, 0, 0, 25);
+	rgba_char_args_set(ts->panelcolors.sub_back, 0, 0, 0, 25);
 
 	rgba_char_args_set(ts->button,         145, 145, 145, 245);
 	rgba_char_args_set(ts->button_title,   0, 0, 0, 255);
@@ -3022,6 +3025,16 @@ void init_userdef_do_versions(void)
 		}
 	}
 
+	if (!USER_VERSION_ATLEAST(280, 17)) {
+		for (bTheme *btheme = U.themes.first; btheme; btheme = btheme->next) {
+			ThemeSpace *ts;
+
+			for (ts = UI_THEMESPACE_START(btheme); ts != UI_THEMESPACE_END(btheme); ts++) {
+				rgba_char_args_set(ts->panelcolors.sub_back, 0, 0, 0, 25);
+			}
+		}
+	}
+
 	/**
 	 * Include next version bump.
 	 */
diff --git a/source/blender/editors/screen/area.c b/source/blender/editors/screen/area.c
index 2491c9f57de..b295d3a0fed 100644
--- a/source/blender/editors/screen/area.c
+++ b/source/blender/editors/screen/area.c
@@ -1806,6 +1806,84 @@ BLI_INLINE bool streq_array_any(const char *s, const char *arr[])
 	return false;
 }
 
+static void ed_panel_draw(const bContext *C,
+                          ScrArea *sa,
+                          ARegion *ar,
+                          ListBase *lb,
+                          PanelType *pt,
+                          Panel *panel,
+                          int w,
+                          int em,
+                          bool vertical)
+{
+	uiStyle *style = UI_style_get_dpi();
+
+	/* draw panel */
+	uiBlock *block = UI_block_begin(C, ar, pt->idname, UI_EMBOSS);
+
+	bool open;
+	panel = UI_panel_begin(sa, ar, lb, block, pt, panel, &open);
+
+	/* bad fixed values */
+	int triangle = (int)(UI_UNIT_Y * 1.1f);
+	int xco, yco, h = 0;
+
+	if (pt->draw_header && !(pt->flag & PNL_NO_HEADER) && (open || vertical)) {
+		/* for enabled buttons */
+		panel->layout = UI_block_layout(
+		        block, UI_LAYOUT_HORIZONTAL, UI_LAYOUT_HEADER,
+		        triangle, (UI_UNIT_Y * 1.1f) + style->panelspace, UI_UNIT_Y, 1, 0, style);
+
+		pt->draw_header(C, panel);
+
+		UI_block_layout_resolve(block, &xco, &yco);
+		panel->labelofs = xco - triangle;
+		panel->layout = NULL;
+	}
+	else {
+		panel->labelofs = 0;
+	}
+
+	if (open) {
+		short panelContext;
+
+		/* panel context can either be toolbar region or normal panels region */
+		if (ar->regiontype == RGN_TYPE_TOOLS)
+			panelContext = UI_LAYOUT_TOOLBAR;
+		else
+			panelContext = UI_LAYOUT_PANEL;
+
+		panel->layout = UI_block_layout(
+		        block, UI_LAYOUT_VERTICAL, panelContext,
+		        style->panelspace, 0, w - 2 * style->panelspace, em, 0, style);
+
+		pt->draw(C, panel);
+
+		UI_block_layout_resolve(block, &xco, &yco);
+		panel->layout = NULL;
+
+		if (yco != 0) {
+			h = -yco + 2 * style->panelspace;
+		}
+	}
+
+	UI_block_end(C, block);
+
+	/* Draw child panels. */
+	if (open) {
+		for (LinkData *link = pt->children.first; link; link = link->next) {
+			PanelType *child_pt = link->data;
+			Panel *child_panel = UI_panel_find_by_type(&panel->children, child_pt);
+
+			if (child_pt->draw && (!child_pt->poll || child_pt->poll(C, child_pt))) {
+				ed_panel_draw(C, sa, ar, &panel->children, child_pt, child_panel, w, em, vertical);
+			}
+		}
+	}
+
+	UI_panel_end(block, w, h);
+}
+
 /**
  * \param contexts: A NULL terminated array of context strings to match against.
  * Matching against any of these strings will draw the panel.
@@ -1815,13 +1893,10 @@ void ED_region_panels(const bContext *C, ARegion *ar, const char *contexts[], in
 {
 	const WorkSpace *workspace = CTX_wm_workspace(C);
 	ScrArea *sa = CTX_wm_area(C);
-	uiStyle *style = UI_style_get_dpi();
-	uiBlock *block;
 	PanelType *pt;
-	Panel *panel;
 	View2D *v2d = &ar->v2d;
 	View2DScrollers *scrollers;
-	int x, y, xco, yco, w, em, triangle;
+	int x, y, w, em;
 	bool is_context_new = 0;
 	int scroll;
 
@@ -1859,6 +1934,11 @@ void ED_region_panels(const bContext *C, ARegion *ar, const char *contexts[], in
 
 	/* collect panels to draw */
 	for (pt = ar->type->paneltypes.last; pt; pt = pt->prev) {
+		/* Only draw top level panels. */
+		if (pt->parent) {
+			continue;
+		}
+
 		/* verify context */
 		if (contexts && pt->context[0] && !streq_array_any(pt->context, contexts)) {
 			continue;
@@ -1920,9 +2000,7 @@ void ED_region_panels(const bContext *C, ARegion *ar, const char *contexts[], in
 
 	BLI_SMALLSTACK_ITER_BEGIN(pt_stack, pt)
 	{
-		bool open;
-
-		panel = UI_panel_find_by_type(ar, pt);
+		Panel *panel = UI_panel_find_by_type(&ar->panels, pt);
 
 		if (use_category_tabs && pt->category[0] && !STREQ(category, pt->category)) {
 			if ((panel == NULL) || ((panel->flag & PNL_PIN) == 0)) {
@@ -1930,56 +2008,7 @@ void ED_region_panels(const bContext *C, ARegion *ar, const char *contexts[], in
 			}
 		}
 
-		/* draw panel */
-		block = UI_block_begin(C, ar, pt->idname, UI_EMBOSS);
-		panel = UI_panel_begin(sa, ar, block, pt, panel, &open);
-
-		/* bad fixed values */
-		triangle = (int)(UI_UNIT_Y * 1.1f);
-
-		if (pt->draw_header && !(pt->flag & PNL_NO_HEADER) && (open || vertical)) {
-			/* for enabled buttons */
-			panel->layout = UI_block_layout(
-			        block, UI_LAYOUT_HORIZONTAL, UI_LAYOUT_HEADER,
-			        triangle, (UI_UNIT_Y * 1.1f) + style->panelspace, UI_UNIT_Y, 1, 0, style);
-
-			pt->draw_header(C, panel);
-
-			UI_block_layout_resolve(block, &xco, &yco);
-			panel->labelofs = xco - triangle;
-			panel->layout = NULL;
-		}
-		else {
-			panel->labelofs = 0;
-		}
-
-		if (open) {
-			short panelContext;
-
-			/* panel context can either be toolbar region or normal panels region */
-			if (ar->regiontype == RGN_TYPE_TOOLS)
-				panelContext = UI_LAYOUT_TOOLBAR;
-			else
-				panelContext = UI_LAYOUT_PANEL;
-
-			panel->layout = UI_block_layout(
-			        block, UI_LAYOUT_VERTICAL, panelContext,
-			        style->panelspace, 0, w - 2 * style->panelspace, em, 0, style);
-
-			pt->draw(C, panel);
-
-			UI_block_layout_resolve(block, &xco, &yco);
-			panel->layout = NULL;
-
-			yco -= 2 * style->panelspace;
-			UI_panel_end(block, w, -yco);
-		}
-		else {
-			yco = 0;
-			UI_panel_end(block, w, 0);
-		}
-
-		UI_block_end(C, block);
+		ed_panel_draw(C, sa, ar, &ar->panels, pt, panel, w, em, vertical);
 	}
 	BLI_SMALLSTACK_ITER_END;
 
diff --git a/source/blender/makesdna/DNA_screen_types.h b/source/blender/makesdna/DNA_screen_types.h
index 7849c05e48b..9213893ae66 100644
--- a/source/blender/makesdna/DNA_screen_types.h
+++ b/source/blender/makesdna/DNA_screen_types.h
@@ -121,7 +121,9 @@ typedef struct Panel {		/* the part from uiBlock that needs saved in file */
 
 	char panelname[64], tabname[64];	/* defined as UI_MAX_NAME_STR */
 	char drawname[64];					/* panelname is identifier for restoring location */
-	int ofsx, ofsy, sizex, sizey;
+	int ofsx, ofsy;                     /* offset within the region */
+	int sizex, sizey;                   /* panel size including children */
+	int blocksizex, blocksizey;         /* panel size excluding children */
 	short labelofs, pad;
 	short flag, runtime_flag;
 	short control;
@@ -129,6 +131,7 @@ typedef struct Panel {		/* the part from uiBlock that needs saved in file */
 	int sortorder;			/* panels are aligned according to increasing sortorder */
 	struct Panel *paneltab;		/* this panel is tabbed in *paneltab */
 	void *activedata;			/* runtime for panel manipulation */
+	ListBase children;          /* sub panels */
 } Panel;
 
 
diff --git a/source/blender/makesdna/DNA_userdef_types.h b/source/blender/makesdna/DNA_userdef_types.h
index 9817d44c96b..468436668b7 100644
--- a/source/blender/makesdna/DNA_userdef_types.h
+++ b/source/blender/makesdna/DNA_userdef_types.h
@@ -150,9 +150,9 @@ typedef struct uiWidgetStateColors {
 typedef struct uiPanelColors {
 	char header[4];
 	char back[4];
+	char sub_back[4];
 	short show_header;
 	short show_back;
-	int pad;
 } uiPanelColors;
 
 typedef struct uiGradientColors {
diff --git a/source/blender/makesrna/intern/rna_ui.c b/source/blender/makesrna/intern/rna_ui.c
index 173eaf56f0c..354fa43367e 100644
--- a/source/blender/makesrna/intern/rna_ui.c
+++ b/source/blender/makesrna/intern/rna_ui.c
@@ -179,6 +179,12 @@ static void rna_Panel_unregister(Main *UNUSED(bmain), StructRNA *type)
 	RNA_struct_free_extension(type, &pt->ext);
 	RNA_struct_free(&BLENDER_RNA, type);
 
+	if (pt->parent) {
+		LinkData *link = BLI_findptr(&pt->parent->children, pt, offsetof(LinkData, data));
+		BLI_freelinkN(&pt->parent->children, link);
+	}
+
+	BLI_freelistN(&pt->children);
 	BLI_freelinkN(&art->paneltypes, pt);
 
 	/* update while blender is running */
@@ -190,7 +196,7 @@ static StructRNA *rna_Panel_register(
         StructValidateFunc validate, StructCallbackFunc call, StructFreeFunc free)
 {
 	ARegionType *art;
-	PanelType *pt, dummypt = {NULL};
+	PanelType *pt, *parent = NULL, dummypt = {NULL};
 	Panel dummypanel = {NULL};
 	PointerRNA dummyptr;
 	int have_function[3];
@@ -229,6 +235,10 @@ static StructRNA *rna_Panel_register(
 				BLI_freelinkN(&art->paneltypes, pt);
 			break;
 		}
+
+		if (dummypt.parent_id[0] && STREQ(pt->idname, dummypt.parent_id)) {
+			parent = pt;
+		}
 	}
 	if (!RNA_struct_available_or_report(reports, dummypt.idname)) {
 		return NULL;
@@ -236,6 +246,11 @@ static StructRNA *rna_Panel_register(
 	if (!RNA_struct_bl_idname_ok_or_report(reports, dummypt.idname, "_PT_")) {
 		return NULL;
 	}
+	if (dummypt.parent_id[0] && !parent) {
+		BKE_reportf(reports, RPT_ERROR, "Registering panel class: parent '%s' for '%s' not found",
+		            dummypt.parent_id, dummypt.idname);
+		return NULL;
+	}
 	
 	/* create a new panel type */
 	pt = MEM_callocN(sizeof(PanelType), "python buttons panel");
@@ -267,6 +282,11 @@ static StructRNA *rna_Panel_register(
 	else
 		BLI_addtail(&art->paneltypes, pt);
 
+	if (parent) {
+		pt->parent = parent;
+		BLI_addtail(&parent->children, BLI_genericNodeN(pt));
+	}
+
 	{
 		const char *owner_id = RNA_struct_state_owner_get();
 		if (owner_id) {
@@ -1102,6 +1122,11 @@ static void rna_def_panel(BlenderRNA *brna)
 	RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL | PROP_ENUM_FLAG);
 	RNA_def_property_ui_text(prop, "Options",  "Options for this panel type");
 
+	prop = RNA_def_property(srna, "bl_parent_id", PROP_STRING, PROP_NONE);
+	RNA_def_property_string_sdna(prop, NULL, "type->parent_id");
+	RNA_def_property_flag(prop, PROP_REGISTER_OPTIONAL);
+	RNA_def_property_ui_text(prop, "Parent ID Name", "If this is set, the panel becomes a subpanel");
+
 	prop = RNA_def_property(srna, "use_pin", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "flag", PNL_PIN);
 	RNA_def_property_ui_text(prop, "Pin",  "");
diff --git a/source/blender/makesrna/intern/rna_userdef.c b/source/blender/makesrna/intern/rna_userdef.c
index fef4cc3ed8e..fc7a0ca6301 100644
--- a/source/blender/makesrna/intern/rna_userdef.c
+++ b/source/blender/makesrna/intern/rna_userdef.c
@@ -902,6 +902,10 @@ static void rna_def_userdef_theme_ui_panel(BlenderRNA *brna)
 	prop = RNA_def_property(srna, "back", PROP_FLOAT, PROP_COLOR_GAMMA);
 	RNA_def_property_ui_text(prop, "Background", "");
 	RNA_def_property_update(prop, 0, "rna_userdef_update");
+
+	prop = RNA_def_property(srna, "sub_back", PROP_FLOAT, PROP_COLOR_GAMMA);
+	RNA_def_property_ui_text(prop, "Sub Background", "");
+	RNA_def_property_update(prop, 0, "rna_userdef_update");
 	
 	prop = RNA_def_property(srna, "show_header", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_ui_text(prop, "Show Header", "");