v50

[B.Douille]

the undo stores all changes even on same adjustment. I found 'undo' confusing, because the tool are applied always in the same order. It's not a stack of actions (you cant have the same adjustment more than once), its to have a real time preview.

I agree, the fact the undo/redo buttons stores all changes step by step accross all adjustments can be confusing. That's why I suggested to highlight the corresponding button for each steps back and forward (the adjustment that was changed at the particular step).
It means, if the user change it all following steps should be cleared.

[xnview]

so for example, we change:
Exposure/Gamma/Saturation. undo => Exposure/Gamma, undo => Exposure, redo => Exposure/Gamma
Exposure/Saturation/Gamma. undo => Exposure/Gamma, undo => Exposure, redo => Exposure/Gamma
or undo => Exposure/Saturation, undo => Exposure, redo => Exposure/Saturation
??

[B.Douille]

Exposure/Gamma/Saturation. undo => Exposure/Gamma, undo => Exposure, redo => Exposure/Gamma

Undo follow the same path, the sequence in reverse order.
Redo re-do the last so still the same path but in same order the user did originally