Created attachment 164302 [details]
screencast of the problem

SUMMARY

I'm reporting this against the Breeze theme because it shows the issue well and degrades the out-of-the-box KDE experience significantly. 

If there's a problem in KWin that's causing this, it may also be noticeable in other themes that like Breeze Light (a) have separate window colors for focused vs unfocused windows, and (b) set the title bar to the same color as the window background in at least one of those states.

STEPS TO REPRODUCE
1. Set your global theme to e.g. Breeze or Breeze Twilight, which should mean you have the Breeze Light color scheme set.
2. Change focus between two windows. It will make the problem much more obvious if you use hover to focus, because focus changing will happen much more often in your routine use of the computer. This *should* be optional though. 

OBSERVED RESULT
Frequently (but not always), the window's title bar will be repainted (to the "inactive" colors) one or more frames earlier than the window background will be repainted. This causes a noticeable flickering / strobing effect that bothers me quite a lot, enough so that I changed my theme colors, and might go so far as to be an accessibility issue for some.

My partner also uses KDE. Their computer is significantly slower than mine and the strobing effect is *greatly* exacerbated by this. To my eye it can be ~0.2 seconds or so before the window colors are back in sync. It's really bad looking. Those on brand new systems with high powered graphics might not notice it, but it seems to be a serious problem for old laptops.

My personal workaround is shown in the screencast. In the first half of the video, I toggle the focus while using a custom color scheme that doesn't change the inactive window colors. Then I switch the colors to Breeze Light and show the bug.

EXPECTED RESULT
The window background should be repainted in the same frame as the title bar 100% of the time with no exceptions.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.90.0
KDE Frameworks Version: 5.246.0
Qt Version: 6.6.1
Kernel Version: 6.6.7-arch1-1 (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION

This isn't a new issue, it's one of those design nits that has bothered me for years and I found a workaround a long time ago (I quit using Breeze colors). I'm reporting it now just because there's a lot of polishing happening for Plasma 6 and I thought someone might want to take a look.