As suggested by Dao in https://phabricator.services.mozilla.com/D9671, this technique could allow us to back out bug 1498356 (which was introduced to work around the fact that -moz-window-titlebar and -moz-window-titlebar-maximized introduce margins on either side of the window). Setting visibility: hidden on the #titlebar, but visibility: visible on its children would mean that we can keep those margins, but we allow any underlying lwtheme textures to show through.

Note that we're already doing this today on Linux, but some patch in bug 1356920 currently moves us away from this. So there isn't really a question whether this technique could work on Linux, and the conservative route here would be not to drop this behavior in the first place.

This allows us to take advantage of the padding that the titlebar gets with the -moz-window-titlebar / -moz-window-titlebar-maximized, but also allows us to bypass the background texture applied to it. We were doing this before, but it got removed in bug 1356920. Bug 1498356 was originally landed as part of a more elaborate solution to this background texture problem, and this patch makes bug 1498356 no longer necessary.

Here's the screenshot comparison: https://screenshots.mattn.ca/compare/?oldProject=try&oldRev=630cd0ecae0a7dedbebc1ff1fe3d4f2c1ccd8e72&newProject=try&newRev=84adb62beb311e998272368eab97ad2106503bfd (my try push also backed out bug 1498356, which I'll also do before landing this)

Please also back out Bug 1498898 which depends on bug 1498356 and you'll get a build failure without it.

Oops, I accidentally triggered landing of this patch and forgot about the backouts. Shouldn't be a problem since this patch should work regardless. Needinfo mconley to take care of the backouts later.

Pushed by dgottwald@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/377c9cbbb5fa Hide the visibility of the titlebar when using lightweight themes with the Linux GTK theme. r=dao

Once this merges to central, I'll back out bug 1498898 and bug 1498356 on inbound.