The bottom overflow image in the frame list is upside down

[captainbrosset]

This happens for me on Windows with Firefox 44.

See this image:

As you can see, the bottom of the frame list is supposed to have a nice overflow image that indicates that there are more frames below, but in my case it looks like it's upside down.

[captainbrosset]

Also (and this could be related to Windows), the overflow image overlaps with the scrollbar.

[juliandescottes]

For your first comment, I would just add this is Firefox only.

The other issue is ... "non-Chrome" specific. Chrome being the only browser where we can easily customize the scrollbars, some absolute positioning & sizes are relying on the scrollbar width which is actually browser specific => let's move this to another issue.