A Qt application of mine began drawing stale text after I scrolled a custom widget. The bug appeared only with desktop compositing enabled, so I immediately blamed the driver. Then I ran it on a different card and saw the same stale text in a slightly different place.

My naive fix was to call update() repeatedly after scrolling. That reduced the frequency without curing it. Repainting more often can hide an invalid region briefly, but it cannot make incorrect geometry correct.

I reduced the widget until it painted only two rectangles and a label. The problem remained when one rectangle moved outside the area returned by the widget’s internal item bounds. My clipping assumptions were wrong: I invalidated the old logical rectangle but painted a pen and text extending beyond it. Compositing changed timing and exposure enough to reveal the mistake more reliably.

After expanding the dirty region to include every painted pixel, both drivers behaved. I then tested without compositing, with XRender compositing and with OpenGL compositing. That matrix is useful because a defect that follows one backend suggests a different layer from a defect that follows the application everywhere.

Adding a plain background behind the moving item made stale areas obvious and gave the reduced test a result anyone could recognize immediately.

Driver debugging still requires exact information. Card family, kernel module, X driver, Mesa version, colour depth and compositor backend can all matter. “Latest driver” is not a version, especially a week later. A minimal program is worth far more than a description of a large application that sometimes looks odd.

The most useful distinction is between wrong commands and wrong execution. If the application issues the wrong clipping, geometry or lifetime operations, changing drivers may only move the symptom. If a tiny correct sequence fails on one driver and succeeds elsewhere, the driver report becomes persuasive.

My practical conclusion is to simplify before assigning blame. Disable layers one at a time, record the exact matrix, and inspect the application’s painted bounds even when hardware acceleration appears implicated. Graphics stacks are complicated enough without accusing the innocent portion first. Unfortunately, every portion has previous convictions.