When documents are placed side by side (creating multiple panes) as below:

And any of the documents are made floating, the empty pane is not removed during LayoutRoot.CollectGarbage:

This works fine if the same thing is done in a floating window. I believe this is due to the fact that for content that is floating during LayoutRoot.GarbageCollect, the PreviousContainer is not cleared. This check does not take into account that the content has already been placed in a floating window when CollectGarbage is executed.