Recent Commits to firefox-csshacks:master

autohide sidebar: Add option for fixed-layout sidebar via pref

autohide-sidebar: make sidebar rules use [sidebar-positionend] attribute

And also remove extra border for revamped sidebar.
This fixes #542

Update menuitem rules to reflect simplifications from 1959540

toolbars below content: use linux top margin hack to extension panels

We already apply computed top margin to notification popup and
permission popup so that they are visible. But it seems that if the
panel is close enough to screen bottom edge (such as in maximized
windows) then the panel contents fail typically fail to render. For
whatever reason, changing the default margin for
customizationui-widget-panel makes the failures much less frequent
although it doesn't appear to get rid of the issue completely.

autohide tabstoolbar: fix positionend selectors for fx138

autohide tabstoolbar: inherit background from sidebar-wrapper