Comment 7 for bug 1963751

Digging a bit further - this machine was manually dist-upgraded on 30-May-2021 (it has -updates enabled, but is set to install only security updates automatically.) That update pulled in libglvnd 1.3.2-1~ubuntu0.20.04.1 (source for libegl1, libglvnd0, etc.)

To upgrade to webkit2gtk 2.34.6-0ubuntu0.20.04.1, u-a must install libopengl0, either 1.3.1-1 from the main archive or 1.3.2-1~ubuntu0.20.04.1 from -updates. However, -updates is not a trusted source for u-a when configured like this, so the only candidate is 1.3.1-1. Unfortunately libopengl0 1.3.1-1 depends on libglvnd0=1.3.1-1, but 1.3.2-1~ubuntu0.20.04.1 is already installed, so u-a would have to downgrade it which is I guess a decision well beyond its pay-grade.

I suppose the solution is to find a way to lose the new dependency, obvious answer would be to backport the fixes in webkit2gtk 2.34.6 to 2.34.4? I assume there is some sort of policy on adding new dependencies in security updates?