Removing the unneeded negative margin trick used to position icons.

This fixes issue #9965.

I also fix #9987 in this pull request since it is showing the left edge of the focus ring with the fix for #9965.
For #9987, it seems like we should just fix the bug that's causing the sidebar to get focus in the first place -- then we wouldn't need this change.

I thought that focus bug (which also occurs on master) was filed, but it looks like I was wrong.  I only see #9414, but that's a lower-priority bug about _temporary_ focus loss when clicking a file -- the issue we're seeing lately is _persistent_ focus loss when clicking a folder.  I think we can just use #9987 to track that issue, since fixing the focus loss itself seems like the right fix for the visible focus ring.