For me it works as long as the viewer is in focus, there is no need to have the slider in focus.
I admit that I’ve also been working with z-stacks a bit recently, and trying shortcuts to change slices because I thought I’d bound it to the scroll wheel… but apparently not. I would also like it connected with scrolling somehow.
I’ve just tried it with the Alt key and I find this appears to block the scroll events (at least on Windows); so does the Shift key. Because Ctrl/Cmd + scrolling is bound to the overlay opacity, I’ve run out of modifier keys to use and can’t think of a way to do it…
Switching to page up/down would be ok; the reason for the arrows is so that left/right can be used to time points as well.
Sidenote: I don’t think QuPath is used a lot with z-stacks, so i’m curious about how you are using it (for me, it’s applying the pixel classifier to confocal stacks). Also, I find the direction of the slider… unintuitive, with slice 0 at the bottom. i wonder if you have also noticed this, and would prefer slice 0 at the top - or a preference to toggle between them?