The issue was that we were misinterpreting the `ANIMFILTER_FCURVESONLY`
flag in the animation filtering code. We were taking it to mean that
literally only actual fcurve channels should be included, but in fact
it should include the channel groups for those fcurve channels as well.
The graph editor code (but not the dopesheet code) depended on this
interpretation to function correctly, and doesn't display the groups
at all without it.
This fixes the issue by not excluding channel groups even when that
flag is set. This also updates the documentation of that flag to
clarify its actual semantics.
Pull Request: https://projects.blender.org/blender/blender/pulls/127108
56e5b61ad0