Hello,
I was wondering it would be cool to have an optional undo function allowing to smoothly transform the paths between the after and before positions, with the same logic and UI as Filters > Transformations > Background.
In other words, to have the possibility to interpolate a path with it’s previous state.
This would require a complete rewrite of the undo system and the app. So its not going to happen, soon.
even as a filter?
Huh? How would that be different to copying the before state into the background (cmd-K), then changing the outline in the foreground, then interpolating with the Transformations filter? How could you implement it with fewer steps?
[Edit: Oops, didn’t read the OP properly.] Agree with mekkablue.
My scenario was: I move my nodes a lot with the alt+cmd keys, let’s say to create a swash.
Then, I realise I moved too much and I’d like something intermediate. Rather than cmd+z and try again, I would apply the fusion undo I suggest. It would just be faster than copying the resulting state, going back, putting -1 step on background, pasting the +1 step to the foreground and applying the filter.
But yes, it can be done with some extra click, loosing a possible pre-existing background.
In fact I use the background to keep some reference shape or an older shape to compare. I find it more practical than to have another layer for temporary variations.
Why not:
1. Copy the current state to the clipboard.
2. Hit Cmd-Z until the foreground has the desired earlier state
3. Paste clipboard contents into the background, go back to the foreground (Cmd-B, Cmd-V, Cmd-B)
4. Interpolate with Filter > Transformations > Background
It does not really matter which is in the foreground, and which is in the background, right?
yes you’re right. You just loose the previous background paths.
Never mind 
The background is meant for operations like this. If you need to store something, copy the layer.
Your suggestion has a problem. How to choose to what state to go back to. One undo or ten, or fifteen? This would require a Time Machine like interface…
And it’s not useful otherwise.