GDKCursor name_fallback is unfortunate and not easy to workaround.
The fallback name map here and on the equivalent wayland code, is a bit unfortunate, since it means that zoom-in
icons, for example, end up displaying the normal icon, if the theme doesn't have that icon present.
This is the case for example for the default Ubuntu cursor theme, which is kind of annoying.
This causes issues with Firefox, as Firefox has its own set of fallback icons to use that are better than the left pointer.
There's a Firefox issue tracking it in https://bugzilla.mozilla.org/show_bug.cgi?id=1428755.
I can work around it, on X11, by calling into X directly (this is on gtk3): https://phabricator.services.mozilla.com/D56361. However on wayland that becomes much harder.
I see that the code for current trunk has changed substantially, and you need to provide the fallback cursor eagerly. That's alright, but unless I'm misreading the code, the fallback cursor still gets checked only after the fallback name, so it still wouldn't work for our purposes...
Is there a way to provide an API to not use the name fallback, or something of that sort?