Firefox has border if started with kwin compositing disabled

[Boris Gjenero]

If I start Firefox with in KDE Plasma with kwin compositing disabled, and the Firefox title bar is disabled via Customize, the whole window has a border. Otherwise my computer is configured to not have window borders, and other applications don't have such a border. I don't know how to make that border go away, besides enabling compositing. Playing with #main-window in userChrome.css shows that the border and margin I can set there exists inside the border which I'm trying to remove.

KWin has a hotkey for toggling compositing, found in Global Shortcuts. Default seems to be Alt+Shift+F12. Toggling compositing doesn't seem to change this immediately. What matters is the state of compositing when Firefox was started.

I'm attaching a screenshot showing this border.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: firefox 85.0.1+build1-0ubuntu0.20.10.1 [origin: Ubuntu]
ProcVersionSignature: Ubuntu 5.8.0-43.49-generic 5.8.18
Uname: Linux 5.8.0-43-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
AddonCompatCheckDisabled: False
ApportVersion: 2.20.11-0ubuntu50.5
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC2: bgjenero 1908 F.... pulseaudio
 /dev/snd/controlC0: bgjenero 1908 F.... pulseaudio
 /dev/snd/controlC1: bgjenero 1908 F.... pulseaudio
BuildID: 20210204182252
CasperMD5CheckResult: skip
Channel: release
CurrentDesktop: KDE
Date: Thu Feb 18 15:15:39 2021
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
IpRoute:
 default via 192.168.1.1 dev eth0 proto static
 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.44
Locales: extensions.sqlite corrupt or missing
MostRecentCrashID: bp-99b8dfa6-8d6d-4541-919c-7d2960210213
PrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/firefox/omni.ja:greprefs.js:353
PrefSources:
 /usr/lib/firefox/defaults/pref/vendor-gre.js
 /usr/lib/firefox/defaults/pref/channel-prefs.js
 prefs.js
 user.js
Profiles: Profile0 (Default) - LastVersion=85.0.1/20210204182252 (In use)
RunningIncompatibleAddons: False
SourcePackage: firefox
SubmittedCrashIDs:
 bp-99b8dfa6-8d6d-4541-919c-7d2960210213
 bp-f8fe1ebe-97ed-4eec-8e60-11f5d0210118
 bp-e9cbbf6c-3c3f-430d-83ce-a0fca0210101
 bp-f2129335-9559-437d-b537-6f4830210101
 bp-70bc911e-707f-4f53-93fe-f2ebf0210101
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: Upgraded to groovy on 2020-10-22 (119 days ago)
dmi.bios.date: 06/19/2009
dmi.bios.vendor: Award Software International, Inc.
dmi.bios.version: F13
dmi.board.name: P35-DS3R
dmi.board.vendor: Gigabyte Technology Co., Ltd.
dmi.chassis.type: 3
dmi.chassis.vendor: Gigabyte Technology Co., Ltd.
dmi.modalias: dmi:bvnAwardSoftwareInternational,Inc.:bvrF13:bd06/19/2009:svnGigabyteTechnologyCo.,Ltd.:pnP35-DS3R:pvr:rvnGigabyteTechnologyCo.,Ltd.:rnP35-DS3R:rvr:cvnGigabyteTechnologyCo.,Ltd.:ct3:cvr:
dmi.product.name: P35-DS3R
dmi.sys.vendor: Gigabyte Technology Co., Ltd.