Show where a blocked drag will land in the Web page designer

When a control is dragged onto cells occupied by another control on a web page, the designer currently commits the drop silently at the next free row, which can place the control much further down the page than expected. The drag gesture should show what will happen before the mouse button is released: while the pointer is over occupied cells, a muted grey dashed outline marks the row the control will actually land on, including its own current row when a blocked drop will leave the control where it was. The design surface also always keeps a little empty grid below the last control so there is always a visible free place to drop at the bottom of the page.