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

[René J.V. Bertin]

Hi,

I am encountering a weird issue that seems to be caused by KWin (X11 AND Wayland).

When session restore is enabled and Firefox configured to show the standard titlebar, windows are downsized vertically each time they're restored after a browser restart. The amount corresponds roughly to either 1x or 2x the height of the titlebar.

This happens no matter how I impose titlebars on FF windows; be it via the browser setting; a special KWin application rule or via `gtk3-nocsd`.

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. 

Anyone else notice this?

R.