Was failing for weightpaint and vertexpaint.
Selection flags were actually changed, but the update in the viewport
wasnt immediate, leading to confusion when the update happened later
(e.g. when using the weight gradient tool as done in the report).
We need to tag ID_RECALC_SELECT and send ND_SELECT notifier here. This
could be done explicitly, but there is also existing functionality
available that does this.
Note: the way updates happen for paintfaces vs. paintverts looks a bit
inconsistent (so this could be part of a later cleanup commit)
Maniphest Tasks: T82881
Differential Revision: https://developer.blender.org/D9631
727d5013a7