Adobe, make the Shift edge option predictable, moving ANY edge by an equal amount.
I decided to check Select & Mask - Shift edge option on 2 simple masks to understand how it actually works.
I can't effectively use this in my workflow, because it has almost no effect on my masks and now it’s clear why...
So, I created two solid color layers, the top one with a mask of 2 simple shapes done with the same Brush, Hardness 0 and 100.


Let's open the mask in Select & mask window and Shift the edge to -100%.

Boooom!

- hard edge has remained almost unchanged, soft edge has become half the size
- both became semi-transparent (not neutral 128, but close to it)

Let's undo and Shift the edge to +100%.

Boooom!

- hard edge remained almost unchanged again, soft edge became twice as large
As we see, the Shift edge option is very specific - it's more effective for the soft edged masks and useless for hard edges... Since I often use the Pen tool for cut outs, I find this a problem. Now I have to modify the mask edge outside the Select & mask window in multiple steps (load a selection from it, then go to the Menu: Select - Modify - Expand/Contract, then fill with white, deselect...). Too many extra steps just because the feature ignores the hard edges... WHY???
SUGGESTION: add a checkbox or another slider which will shift ANY edge by an equal amount.
Just like the Menu command Select - Modify - Expand/Contract. I don't wanna make those extra steps, I want it work the same predictable way here, in Select & mask workspace.
P.S. While writing this post and playing with other sliders I've found a solution to affect the hard edged mask. But it's not so obvious and anyway requires extra unnecessary clicks.

In practice we can expand the hard edged mask with... the Feather slider! (it's not obvious, I told you 🙂 LOL). At the same time the Contrast must be 100%, the Shift edge value may vary. For simple masks it may work, but for complex ones, I doubt.... And this experimenting with a few sliders to change one parameter is nonsense! Not handy at all.