In gtk testing we are using Xvfb and software rasterization for layout test, this is detected to use the X11 backends and GLX in WebKit, we need to make sure we have the GLX backend compiled with ANGLE and we use it when we are in DEVELOPER_MODE in the fallback class.
Pull request: https://github.com/WebKit/WebKit/pull/4429
After https://commits.webkit.org/254751@main this is not a problem anymore because we prefer EGL, so we go in the good directorion to remove GLX. I'm closing this PR.