For anyone interested, it seems that at least on Linux, the best/easiest way to make sure QtWebEngine's chromium/blink uses 100% software rendering (in Qt6.9.0+) is to set the QT_QUICK_BACKEND environment variable to "software" (without the quotes, of course).
On my system, this bypasses the attempt to use GBM, AND the fallback to Vulkan. There's no noticeable performance degradation for me when running Sigil in this way (but my hardware is pretty beefy).
I would recommend that anyone experiencing Sigil Preview problems/sluggishness on Linux (using Qt6.9.0+) at least
try launching Sigil from a terminal with:
Code:
QT_QUICK_BACKEND=software sigil