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The walls of this box are separate objects attached to the box. Previously, if a wall was bumped, it just didn't move because the wall wasn't pushable. Now, if a box wall is bumped, it refers the bump to the object its attached to.
In this case, that's the box which is pushable. So, the box moves, and it already had code that moves all of things attached to it (the walls). It was surprisingly little code, and I learned that GameMaker supports weak references.