Testing with Ubuntu MATE 18.04(.1) using the default wm/compositor Marco. I've tried Nightly (downloaded from Mozilla), Firefox 61 (the version in the Ubuntu repo), and back to 60 (downloaded from Mozilla) (I think the first version with CSD?). - With Customize → Title Bar unchecked to enable CSD, make sure Firefox is maximized, then close it. - Reopen Firefox, and then unmaximize the window. An extra titlebar and border now appears with a transparent border around Firefox window, as show in the first screenshot. - When you maximize the window, the border goes away, but the extra titlebar remains, as shown in the second screenshot. - If you close Firefox when it is *not* maximized and reopen it, then it will behave correctly even when maximizing and maximizing — until you close it again maximized and reopen it. It does not happen every time, i.e. sometimes you can close Firefox maximized and it will behave correctly, but it occurs more often than not. I've tried it on several computers, trying to find a pattern, but I can't find one... It does seem that it happens more often on a computer with HiDPI, but I've only tried one computer like that. The strangest part is that it doesn't occur at all on my personal computer's main user account, even if I create a new Firefox profile. I mostly use Nightly, so when I first saw it on other computers, I thought it was something that had been fixed between the release version and the Nightly version I was using ;-) But it was reported on the Ubuntu MATE forum, and now I've tried it more and found that it does occur when I create a new user account on my computer, and I've seen it on every other Ubuntu MATE installation I've tried, even with Nightly. (I'm have no idea what setting/tweak I have in my user account that could be affecting it!) I'm happy to help testing if you let me know how. I can also report it to MATE/Marco if you think that's where the issue is. Thanks for all the work on Firefox CSD!
I experience the same problem with gnome-flashback, see my comment here: https://bugzilla.mozilla.org/show_bug.cgi?id=1408360#c32 Maybe this is somehow related to Bug 1452520?
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