See #5888. Fix bug where loading type thumbnails made undo history not synced with Blender.

If a tool.Ifc.Operator updates a prop, the prop update function will
only call _after_ the operator finishes (and therefore adds an undo step
to the undo stack). If the prop update function then calls another
tool.Ifc.Operator, that will result in another "top-level" operator
call. This second operator _won't_ get added to Blender's undo history,
yet the Bonsai history / IfcOpenShell history will have another undo
step added. Yikes!

TL;DR don't call tool.Ifc.Operator from a prop update function.

src/bonsai/bonsai/bim/module/model/product.py

@@ -618,7 +618,7 @@ class AlignProduct(bpy.types.Operator):
         return results
 
 
-class LoadTypeThumbnails(bpy.types.Operator, tool.Ifc.Operator):
+class LoadTypeThumbnails(bpy.types.Operator):
     bl_idname = "bim.load_type_thumbnails"
     bl_label = "Load Type Thumbnails"
     bl_options = {"REGISTER", "UNDO"}
@@ -626,9 +626,9 @@ class LoadTypeThumbnails(bpy.types.Operator, tool.Ifc.Operator):
     limit: bpy.props.IntProperty()
     offset: bpy.props.IntProperty()
 
-    def _execute(self, context):
+    def execute(self, context):
         if bpy.app.background:
-            return
+            return {"FINISHED"}
 
         props = tool.Model.get_model_props()
         # Only process at most one paginated class at a time.