Dropping an object between two GridPane rows does not reorganize the grid content correctly

1) Open (atttached) LittleGridPane.fxml
2) Drag a Button from Library to the GridPane between row 0 and 1
    => this inserts a new row after row #0 : OK
    => this adds new Button to (new) row #1 : OK
    => this moves button B2 to row #2: OK
    => this leaves button B3 in row #2: BUG
         => B3 should have moved to row #3