Hmmm so this is new. Kwin-wayland 5.22 is now giving me the same glitched pixel (and a few other display oddities) that I consistently see in the Sway window manager but seemingly only when I change "Adaptive Sync" from "Automatic" to "Always".
Wonder if it's the code or monitor.
GNOME Shell does not have this problem but I suspect GNOME Shell might not fully enable Adaptive Sync/Variable Refresh Rate in multi-display setups like mine.
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