On my Windows machine (Version 1803, OS Build 17134.5), when maximizing windows tabs are no longer aligned to the top of my screen (150% dpi, 3840 x 2160). I can reproduce on both 60b16 and current nightly. Looking at the layout in the inspector, it looks like what's happening is the -moz-window-titlebar-maximized causes the titlebar to have a height of 33 and padding of 8.6667, whereas it gets a negative margin of 33, so there's an 8.6667 css pixel gap.

As noted on IRC, I can't reproduce the issue with the same resolution and 200% scaling, so the rounding theory looks pretty plausible. Probably too late to fix this for next week's Fx60 release, but we should keep this on the radar for any possible dot releases down the road.

OK, so the weird thing is that visually the gap isn't 8.6 pixels. The gap is more like 1-2 pixels. It looks like when maximized, window.screenX is -7 (on my machine, anyway), so presumably the actual visual gap is only about that. So compensating for the full moz-appearance-induced padding would also be wrong... Jonathan, I don't suppose you have any ideas on how I can debug this further? Feels like the values we get here: https://searchfox.org/mozilla-central/rev/8837610b6c999451435695e800f38d4acbc0a644/widget/windows/nsNativeThemeWin.cpp#2065-2075 must be wrong, but I dunno why that would be the case...

This is probably a dupe of bug 1403153. Gijs, can you check the screenshot in that bug and dupe it if so? :)

(In reply to Johann Hofmann [:johannh] from comment #3) > This is probably a dupe of bug 1403153. Gijs, can you check the screenshot > in that bug and dupe it if so? :) Gah. I spent a good 10 minutes looking for that bug, which I was sure existed, and didn't find it. :-\ I'll dupe this over. Also, miraculously, this has gone away for me after a restart...

To be clear, the spacing wasn't as extreme as in that bug, and it wasn't differently-coloured, but I expect it's the same underlying issue.