System Information Operating system: Windows-7-6.1.7601-SP1 64 Bits Graphics card: GeForce GTX 980 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 391.35 Blender Version Broken: version: 2.91.0, branch: master, commit date: 2020-11-25 08:34, hash: rB0f45cab862b8 Worked: 2.79 Short description of error Weight Paint Redo Panel Undoes Too Many Steps Exact steps for others to reproduce the error

Load WeightPaintRedoPanelProblems-279vs291.blend

in 2.91=====================

Select the mesh.

1: Edit mode, select vertices in the center of the chest
2: Switch to Weight paint mode
3: Turn on Vertex Masking Mode*
4: Select vertex group -- one that is not selected, example: mChest*
5: Menu: weight Smooth
6: Redo Panel -- try to change iterations and/or expand/contract

RESULT: the vertex group you selected is no longer selected. vertex masking mode is no longer on. the redo is not acting on what you wanted it to act on.

REASON: the redo panel performs an undo when you try to change its controls. steps marked with * are not recorded for undo/redo.

2.91's Undo History List (compared to the steps taken)
5: Menu: weight Smooth
2: Switch to Weight Paint Mode

FIX:====================

you must toggle out of, and back into, weight paint mode, so that your vertex masking mode change and vertex group selections are before the redo panel's undo step.  ie:

actions:
1-5: as above
6: edit mode
7: weight paint mode
8: Menu: weight smooth
9: Redo Panel

undo list:
9: weight paint smooth
7: weight paint mode

for comparison:
BLENDER 2.79=============

actions:
1: Edit mode, select vertices
2: Weight Paint Mode
3: Vertex masking Mode
4: Select vertex group
5: Weight Tools: Smooth
6: Redo Panel

RESULT: it functions on the selected vertices/vertex group as expected.

undo list:
5: Smooth Vertex Weights
4: Select Vertex Group
3: Vertex masking Mode
2: Weight Paint Mode
1: Toggle Edit Mode