Open MiniBrowser (or Epiphany), launch five browser tabs, and use gnome-system-monitor to inspect the open files of the first and last WebKitWebProcess. Do not use flatpak, because when running Epiphany under flatpak, everything seems fine for some reason I don't understand. But when running outside flatpak, I see the number of local sockets usually -- but not always -- increases by 2 for each web process launched. We are leaking some file descriptors... somewhere. Sadly, this is going to be hard to debug.
From bug #220090, we can probably avoid this by removing G_SUBPROCESS_FLAGS_INHERIT_FDS, but to do that we need a way to exempt the WPE render host socket by passing it to ProcessLauncher and from there to g_subprocess_launcher_take_fd(). r241816 -- which seemed like a nice cleanup at the time -- made this harder.
I tracked this down to wpebackend-fdo, https://github.com/Igalia/WPEBackend-fdo/pull/136.