Part of #107742.
There used to be a small margin between items since the layout feels a
bit crammed otherwise. But this meant the mouse could be between items,
with no item highlighted or reacting to interactions. This was
especially annoying when dragging over items for drag and drop: in
between items dropping wasn't possible, and the drag-tooltip would
disappear, causing notable flickering during motions over the tree view.
The view item is now slightly enlarged to keep a look that is not too
crammed, and still remove the space between items. Item highlights are
still drawn with a smaller height (matching the normal widget height),
since anything else looked odd to me.
This now feels quite consistent with similar UIs (e.g. File Browser list
view or the Outliner), even though we give the items a bit more space.
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