Bug 489838

Summary:	[wayland] If panel opacity changes when switching virtual desktops, the panel glitches while the opacity changes
Product:	[Plasma] kwin	Reporter:	Eamonn Rea <eamonnrea>
Component:	compositing	Assignee:	KWin default assignee <kwin-bugs-null>
Status:	REPORTED ---
Severity:	normal
Priority:	NOR
Version First Reported In:	6.1.2
Target Milestone:	---
Platform:	Other
OS:	Linux
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:

Description Eamonn Rea 2024-07-06 17:21:02 UTC

SUMMARY
If panel opacity changes when switching virtual desktops, such as going from a virtual desktop with a window maximised to an empty virtual desktop with with no windows, the panel will glitch while the opacity adjusts.

This happens when switching virtual desktops using a keyboard shortcut, but seems to be more prevalent when switching using a touchpad gesture. This is probably because the speed can vary. Depending on how you perform the gesture sometimes it is not visible, but performing the gesture very fast makes the glitching very noticeable. The effect is still noticeable when switching with a keyboard shortcut but it happens earlier.

It *seems* like the opacity smoothly transitions if you perform the touchpad gesture to move around halfway between the virtual desktops and let go, then the opacity smoothly transitions and does not glitch. It is somewhat precise, as too much / too little still causes the glitching. I don't have a touchpad on my PC to test there.

I have my panel at the top of the screen, but this also occurs with panels at the bottom of the screen. I did not test if this happens with panels to the left / right of the screen.

This happens on my Desktop PC and my laptop, using the open source Mesa drivers. There is potential it could be a GPU driver bug but if so it would be specific to AMD / Mesa. I do not have any Intel or Nvidia hardware to test on.

I don't believe bug was present in Plasma 5, but it is new to Plasma 6, and I am unsure when it started happening in Plasma 6.

STEPS TO REPRODUCE
1. Have a panel with adaptive opacity, and at least two virtual desktops
2. Maximise a window on one of the virtual desktops
3. Switch virtual desktops with a keyboard shortcut OR a very fast / very slow touchpad gesture (sliding with three fingers).
4. Observe that the panel glitches briefly while it settles into the correct opacity.

OBSERVED RESULT
Panel opacity glitches if it changes while switching virtual desktops.

EXPECTED RESULT
Panel opacity should not glitch and should smoothly transition.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.9.7 Linux Zen
KDE Plasma Version: 6.1.2
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.2

ADDITIONAL INFORMATION