Created attachment 95729 [details]
Example. Look at the shadow of "Window" menu
Shadows drawn by oxygen-gtk theme engine look pixelated. Example is attached.
On UXA backend shadows are smooth.
Tried both 2.99.910 and git master.
Xorg server 1.15.0
I think this will be an issue with the precision of the accumulation buffer as ana tries to do this in hardware. A full-debug will confirm my suspicions.
Details, details, details. If you force me use KDE, you should at least tell me how to configure it to reproduce your effect.
If you need details, tell me please, what details do you need.
You don't need KDE to reproduce it, I think.
You could install gtk2-engines-oxygen on debian or oxygen-gtk2 on fedora or package with almost same name on almost any distro, set "oxygen" theme for gtk, and look at shadows of popup menus or tooltips.
At least, I am sure this problem is present on debian sid (if I rebuild the driver with SNA enabled) and current arch linux.
No special configuration is needed. Only oxygen-gtk and SNA.
Looked at the source code for oxygen-gtk and they are not using convolutions but gradients. The result is just the precision of your GPU to compute an alpha gradient.
To reproduce it, additionally to oxygen-gtk+gtk app, you'd need any of the following, because the shadows are drawn by window manager, and oxygen shadows are only supported by oxygen-enabled WM:
1. kwin with KDE oxygen theme (KDE window manager)
2. patched (as per ) XFWM4 (XFCE window manager) with oxygen theme from 
3. oxygen-gtk-deco, obtainable from building oxygen-gtk2 from source, passing -DBUILD_DECO_TEST=1 to cmake to build a test app for window decoration rendering, then starting oxygen-gtk-deco and trying to resize the solid window (the one without hole) to get inactive shadow.
I didn't test though since I don't have latest intel driver.