UI: Avoid rebuilding Outliner tree when opening/collapsing items

In big files, the Outliner would have a noticeable lag when opening or
collapsing items. That was because the entire tree was rebuilt, which isn't
actually needed in most cases. So we avoid it where possible now.

source/blender/editors/space_outliner/outliner_edit.c

@@ -191,7 +191,14 @@ static int outliner_item_openclose_modal(bContext *C, wmOperator *op, const wmEv
       /* Only toggle openclose on the same level as the first clicked element */
       if (te->xs == data->x_location) {
         outliner_item_openclose(te, data->open, false);
-        ED_region_tag_redraw(region);
+
+        /* Avoid rebuild if possible. */
+        if (outliner_element_needs_rebuild_on_open_change(TREESTORE(te))) {
+          ED_region_tag_redraw(region);
+        }
+        else {
+          ED_region_tag_redraw_no_rebuild(region);
+        }
       }
     }
 
@@ -231,7 +238,13 @@ static int outliner_item_openclose_invoke(bContext *C, wmOperator *op, const wmE
                       (toggle_all && (outliner_flag_is_any_test(&te->subtree, TSE_CLOSED, 1)));
 
     outliner_item_openclose(te, open, toggle_all);
-    ED_region_tag_redraw(region);
+    /* Avoid rebuild if possible. */
+    if (outliner_element_needs_rebuild_on_open_change(TREESTORE(te))) {
+      ED_region_tag_redraw(region);
+    }
+    else {
+      ED_region_tag_redraw_no_rebuild(region);
+    }
 
     /* Only toggle once for single click toggling */
     if (event->type == LEFTMOUSE) {

source/blender/editors/space_outliner/outliner_intern.h

@@ -228,6 +228,8 @@ void outliner_build_tree(struct Main *mainvar,
                          struct SpaceOutliner *soops,
                          struct ARegion *region);
 
+bool outliner_element_needs_rebuild_on_open_change(const TreeStoreElem *tselem);
+
 typedef struct IDsSelectedData {
   struct ListBase selected_array;
 } IDsSelectedData;

source/blender/editors/space_outliner/outliner_tree.c

@@ -235,6 +235,17 @@ static TreeElement *outliner_add_element(
 
 /* -------------------------------------------------------- */
 
+/**
+ * Check if an element type needs a full rebuild if the open/collapsed state changes.
+ * These element types don't add children if collapsed.
+ *
+ * This current check isn't great really. A per element-type flag would be preferable.
+ */
+bool outliner_element_needs_rebuild_on_open_change(const TreeStoreElem *tselem)
+{
+  return ELEM(tselem->type, TSE_RNA_STRUCT, TSE_RNA_PROPERTY, TSE_KEYMAP);
+}
+
 /* special handling of hierarchical non-lib data */
 static void outliner_add_bone(
     SpaceOutliner *soops, ListBase *lb, ID *id, Bone *curBone, TreeElement *parent, int *a)
@@ -786,8 +797,13 @@ static void outliner_add_id_contents(SpaceOutliner *soops,
   }
 }
 
-// TODO: this function needs to be split up! It's getting a bit too large...
-// Note: "ID" is not always a real ID
+/**
+ * TODO: this function needs to be split up! It's getting a bit too large...
+ *
+ * \note: "ID" is not always a real ID
+ * \note: If child items are only added to the tree if the item is open, the TSE_ type _must_ be
+ *        added to #outliner_element_needs_rebuild_on_open_change().
+ */
 static TreeElement *outliner_add_element(
     SpaceOutliner *soops, ListBase *lb, void *idv, TreeElement *parent, short type, short index)
 {