My issue appears to be largely due to a version mismatch, and I have been able to resolve this.
I had tried to get a specific version from unstable to test a fix for a different bug, but apparently I misunderstood how pinning works and got updates to mesa packages from the unstable release even after the version for which I intended this. Once I downgraded mesa packages to testing version, I was able to start X again. I'll leave it to you whether you believe this kind of issue is worth addressing at the distro level (perhaps via more strict version dependencies?) or forwarding to the upstream maintainers (so that unexpected behavior from a module library gets detected in a nicer fashion than by crashing the X server). If you decide to just close this bug, that's fine with me, too. In that case sorry for the noise. $ cat /etc/apt/preferences.d/mesa.pref Explanation: test fix for #933906 Package: *mesa* Pin: release a=unstable Version: 19.1.4-1 Pin-Priority: 600 $ dpkg -l '*mesa*' Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad) ||/ Name Version Architecture Description +++-===============================-============-============-=========================================================> ii libegl-mesa0:amd64 19.2.6-1 amd64 free implementation of the EGL API -- Mesa vendor library ii libegl-mesa0-dbgsym:amd64 19.2.6-1 amd64 debug symbols for libegl-mesa0 ii libegl1-mesa:amd64 19.3.1-3 amd64 transitional dummy package ii libgl1-mesa-dri:amd64 19.3.1-3 amd64 free implementation of the OpenGL API -- DRI modules ii libgl1-mesa-dri-dbgsym:amd64 19.3.1-3 amd64 debug symbols for libgl1-mesa-dri un libgl1-mesa-glx <none> <none> (no description available) un libgl1-mesa-swx11 <none> <none> (no description available) ii libglapi-mesa:amd64 19.3.1-3 amd64 free implementation of the GL API -- shared library ii libglapi-mesa-dbgsym:amd64 19.3.1-3 amd64 debug symbols for libglapi-mesa un libgles2-mesa <none> <none> (no description available) ii libglu1-mesa:amd64 9.0.1-1 amd64 Mesa OpenGL utility library (GLU) ii libglx-mesa0:amd64 19.3.1-3 amd64 free implementation of the OpenGL API -- GLX vendor libra> un libwayland-egl1-mesa <none> <none> (no description available) un mesa-opencl-icd <none> <none> (no description available) un mesa-utils <none> <none> (no description available) ii mesa-va-drivers:amd64 19.3.1-3 amd64 Mesa VA-API video acceleration drivers ii mesa-vdpau-drivers:amd64 19.3.1-3 amd64 Mesa VDPAU video acceleration drivers ii mesa-vdpau-drivers-dbgsym:amd64 19.3.1-3 amd64 debug symbols for mesa-vdpau-drivers ii mesa-vulkan-drivers:amd64 19.3.1-3 amd64 Mesa Vulkan graphics drivers un mesag3 <none> <none> (no description available) un xlibmesa3 <none> <none> (no description available)
