Working set selection pointer overlaps file tree top highlight

[njx]

1. Open a lot of files in the working set
2. Select a file in the middle of the working set list
3. Scroll the working set so the file selection highlight crosses the bottom boundary

Result: The right edge of the highlight line at the top of the file tree is overlapped by the file selection pointer. Probably a z-index issue. See the image below.

[njx]

This isn't related to themes - it repros in Release 41 I think. Might have been in there for awhile, although I'm surprised we never noticed it before.

[njx]

Marking F Project Management since this is related to the sidebar, although I'm not sure if the working set UI counts as part of that...

[JeffryBooher]

We should wait until after Splitview lands to fix this. Since it's low priority it shouldn't be a problem.

[villimagg]

As y'all can see there's an rendering issue for Linux users. I don't know if this is related to extensions or the UI of the editor since I installed Brackets and started installing extensions right away.