Widget Editor: Drop placeholder should appear below any existing blocks/widgets

[mapk]

Current in #25681, when hovering over a block in the Inserter, the drop placeholder (blue line) appears at the top of the first widget area when nothing is selected. It appears at the top even when there are existing blocks/widgets already in that area.

I'd like this to mimic the behavior in the post editor. This means that the blue line should appear below any existing blocks/widgets.

If the area has so many blocks in it that it extends below the screen, it should follow the pattern found in the post editor which is to not scroll to show the placeholder, but instead drop the block in the area and then move that area into view with the block selected.

cc @adamziel @draganescu