The workaround for this glitch has been implemented in v1.26.4.5.
This glitch is actually due to rigged mesh attachments rebuilds: it still exists (in all mesh viewers) and can still be seen happening, sometimes, when zooming in/out (LOD changes trigger rigged mesh attachments rebuilds) and during rigged mesh initial rezzing (see
SH-3013).
The work-around I implemented simply consists in not rebuilding rigged meshes every few frames as long as you have the Mesh Deformer disabled in your viewer or, when you have it enabled, as long as the avatars wearing rigged meshes do not use any deformer-ready mesh.
It means that if an avatar wears a deformer-ready rigged mesh attachment and you have mesh defomer active in your viewer, you may still see rigged meshes "jumping" on that avatar (and only on that one) if the meshes they wear are sensitive to this issue (*). Also, if you activate the "Force Mesh Deformer" advanced setting (something that should not be done but for debugging and testing), then the work around I implemented will be bypassed and you will see the "jumping" happening for all avatars wearing affected rigged mesh attachments (*).
I posted a report on Qarl's blog to make him aware of this issue, but the true fix for this glitch is to have
SH-3013 fixed (please, vote for it).
(*) affected rigged meshes are the ones with a non-zero offset.