Panels with a zero dimension are intended to be unclipped in that dimension, if I understand the docs and the editor output for how it draws the purple squares.
If that is the case, should UIDraggablePanel::shouldMove be sensitive to zero sizes? Currently, if you have a panel with a zero size, and set UIDraggablePanel::disableDragIfFits to true, the panel will still move around if the contents fully fit in the panel.