Drop on Grid not working

I want to drag an item from one grid to another grid. It worked in earlier versions. However, with 14.1.16 and 14.1.17 it does not work. The GridDropEvent is not fired. Also, it does not show any blue frame around the grid. DragStart and DragEnd events from the source grid are fired.

Any idea?

private class MyGrid extends Grid<MyType> {
	public MyGrid() {
		this.addDropListener(event -> {
			// does not fire!

Caused by me. I had a z-index set on the table and tbody of the grid. I had to use this in order to make tooltips show.