I'm using Darktable to edit photos. There is one option, which I'm still looking for, but it seems to be unavailable or hidden somewhere. How to (after making some changes) preview a source image in order to compare before & after versions?
It's possible to turn on and turn off one separate function (small power off button next to an option), but how to switch between before/after without clicking each of them separately?
Have a look at snapshots, top left in the darkroom mode. For details, have a look at https://www.darktable.org/usermanual/en/snapshots.html
You can click on one step of the history stack on the right hand side to view the image "as it is at that step". Click back to the top to view the latest version. Warning: while you are viewing one step of the history stack, any attempt to edit the picture will discard the top of your stack.
You can go to the step you want in history, make a snapshot, come back to the top of history, and compare with the snapshot.