Consider the following case:

Create a grid with two rows.
Place a button in the first row.
Increase the row span of the button to 2
Move the button in the second row: a third 'ghost' row appear.

This is conform to the GridPane API/spec - however the visual effect is strange.

In all cases we should try and avoid a new row/column being unexpectedly created in the GridPane.

If the user moves an element to a row below its current position and the selected child has a row span that exceeds the remaining number of rows then the rowspan value should be decreased accordingly.

When we reject adjust a rowspan maybe we should show a message?