Here is a capx where you can recreate it.
To test it, press the "Generate large dungeon" and while it is generating (The background is still white) press Pause in the debugger.
In the left select the "List_dungeon_possible_tiles".
Then minimize the window and maximize it again. And the items in the list have changed. If you keep minimizing and maximizing again you can see it actually drawing the dungeon each time.