I think the issue is that the run-node-group operator overwrites the data stored in the
`Mesh` while mesh edit mode operations typically only change the `BMEditMesh`. It
seems like that causes issues because the mesh edit mode undo stack does not
keep track of changes to the `Mesh`. When hitting undo, Blender assumes that the
`Mesh` stored in the object has not changed and therefore it does not have to be
read from the undo step again.
The preliminary fix implemented here is to just not change the `Mesh` but only the
`BMEditMesh` like any other edit mode operator. This seems to solve the issue.
I haven't quite figured out yet how to tell the undo system that the `Mesh` has to be
loaded from the undo step when undoing out of mesh edit mode. Doing that might
provide a better solution.
Pull Request: https://projects.blender.org/blender/blender/pulls/119015
Jacques Lucke
b40a9ce60b