After a driver update, newly opened windows sometimes became black under KWin’s OpenGL compositor. The same windows were fine with XRender. Turning off every effect changed nothing, which at least got the animations out of the suspect list.

The useful split was the texture-from-pixmap path. Under X11 compositing, the server redirects a window into a pixmap. KWin’s OpenGL path then uses the driver’s texture-from-pixmap support to bind that pixmap as a texture for the scene. XRender composites the pixmap through a different route. Applications had painted the right pixels; the OpenGL path was failing to present them.

I could make it happen by opening two terminals, covering one, and uncovering it. Damage arrived, but the exposed window stayed black or returned in strips. Switching to the previous driver fixed that exact sequence. The newer driver also behaved if I disabled its acceleration option, slowly.

That made a much better report than “KWin draws black windows.” I included the X server, Mesa, kernel and driver versions, the OpenGL backend, hardware identifiers, and the short cover/uncover sequence. I also noted that XRender worked. That last fact tells a driver developer far more than a screenshot of a black rectangle.

Screenshots were awkward: some captured the correct window even while the display showed black. The redirected pixmap could be fine while the texture or final scanout was stale. A camera photo was ugly but honest.

I am using XRender for normal work and keeping the failing setup available for testing. Reinstalling KWin would only replace the code that asks the driver to bind the same pixmap. The break is farther down the path, and now there is a reproducible way to reach it.