#109460 was caused by the introduction of implicit sharing for custom
data layers (7eee378eccc8f87e17). Due to implicit sharing, the vertex
weight layer is now shared between the original and evaluated object in
object/weight paint mode, but not in edit mode because of bmesh. For
that reason, the original data has to be copied to be able to edit it, which
causes its pointer to change. `uiDefButF` can't deal with the situation when
the pointer changes on every redraw.
The solution is to just use an intermediate variable that is passed to the
`uiBut` and to update the vertex weight only when the intermediate value has
changed.
Pull Request: https://projects.blender.org/blender/blender/pulls/109665
cd74ba6413