If you focus an item, you can resize it with "Ctrl+Arrow keys". If you for example press "Ctrl+Arrow up", the items vertical size shrinks. If you do that repeatedly, it becomes less than 0. If you want to increase the size again, you first have to undo all that negativity, before the items grows again. Ideally, the code should only reduce "ColSpan" and "RowSpan" if its current value is greater than 1.

The EventCallbacks "OnResize" and "OnReorder" only get invoked when a user is performing the appropriate actions with the mouse, not when using keyboard navigation.