KWin causing Firefox windows to be downsized upon restart with session restore

[René J.V. Bertin]

On Tuesday April 28 2026 13:51:34 Duncan wrote:

>> I thought this was a pure Firefox bug, but it doesn't happen when I run
>> Xfwm4 (under a KDE environment).
>> 
>> I've now seen it in KWin4 and KWin5 5.15.5 and whatever (quite) newer
>> version is in Devuan Chimaera.
>> 
>> It only happens though after I exited the browser properly, not after an
>> unexpected exit.

Wow, as usual with your replies, it's a real question where to begin to answer! :)

Re: libglvnd : I don't even have that installed.

Re: windecos in KWin: in KWin4 I use the one from QtCurve. That was sadly never ported to KF5 so with KWin5 I use the Oxygen deco there. With small buttons and a 10pt titlebar font (@86DPI) I get titlebars that are compact enough even on a 1366x768 LCD panel.

I see I didn't mention it but the issue occurs for me under Wayland too (the KF5 install in Devuan Chimaera is recent enough to provide a stable Wayland environment). On a side-note: with KWin5's continued support for decoration themes I think we must be talking about server-side decorations anyway (hooks in the platform plugin to theme CSD would only work for Qt-based applications).

But the crucial point here is that it is really something specific to Firefox's session restore feature, and only when it's allowed to do its work properly (or the way it thinks proper).

It looks like something in the code gets executed when doing a proper "quit" updates the actual positions and sizes of the windows that are to be restored, and makes an error. Prevent that code from being executed by killing the application and windows do return to their expected positions and sizes.