My issue is successfully resolved. I’m posting because I was not happy with my ability to find helpful descriptions of the problem.
Posting also to ask where I can report it, and where I should check for similar reports when encountering similar issues in the future.
Hardware: MacBookPro11,5
GPU: AMD ATI Radeon HD 8870M / R9 M270X/M370X
Kernel: 6.5.0-40-generic, fixed by booting in: 6.5.0-45-generic
I observed an issue where 5/6ths of the desktop background was white or grey, not rendered, from initial login at the sddm prompt to the desktop. Once logged in, many windows including for KDE settings and yakuake, though not for browsers or electron apps, had large blank white patches.
- In settings, a patch in the upper right, was not rendered.
- In yakuake, large sections of blank black terminal, or terminal with uniform/repeating content, would not be rendered, instead showing as blank white patches. Individual lines or portions of lines containing characters would render and appear for a time, and selecting text would make a section visible, until clicking away made it invisible again (KWin grey on background window setting set).
It affected the laptop screen, and an external display. The issue was not visible on alternate ptys, but did survive reboots. I did also notice a color inversion in the browser, affecting a matching top 7/8ths of the new tab background window.
I was worried this was a hardware issue, first signs of a GPU dying. Fortunately, it appears it was a software & kernel version mismatch issue. I found talking to Claude helpful, first with the issue I noticed, then completing more of my message with useful context – asking for clarifying questions, ways to diagnose & to distinguish different possible issues, and for fixes.