Reasoning:
- Different Linux DEs specify different titlebar button (min, max,
close) locations which we did not and could not support before.
- Previous approach copied from Firefox depended on custom offsets
specified in CSS instead of allowing the browser engine to place
elements
Changes:
- Titlebar and menubar no longer collapsed into each other with a
negative margin, instead just a single flexbox (on macOS the titlebar
is still collapsed into the tab bar)
- Support for -moz-gtk-csd-reversed-placement to correctly place the
titlebar buttons on Linux
- Removed/changed some code copied from Firefox that does not apply or
uses the wrong selectors
- Fixed the display of Linux Unity DE buttons with icons appearing outside
of button borders
- Removed the Z in the titlebar on Linux since only a few DEs use that.
- Aligned the tabbar on Linux and Windows to the left margin of the
window.
