Currently, no change to an RNA property invoked from the UI while inside
Weight Paint mode creates an undo step. This includes both tool
properties (e.g. brush size, tool settings) as well as mesh properties
(symmetry, active vertex group, selection mode).
This behavior was initially introduced to avoid creating extra undo
steps when painting or sculpting and changing tool settings, however,
it applied to all property changes performed while in any paint mode,
including changes to mesh datablock properties.
To fix this behavior, this commit simply removes Weight Paint from the
list of modes that have their own (legacy) undo system. This means it's
now covered by the regular memfile / global undo system.
This builds on !134548 and !134836.
Ref: #134220, #69760, and many others
Pull Request: https://projects.blender.org/blender/blender/pulls/135758
96869d03d4