In practice arrow keys would only work if the user has already clicked on a knob in the plugin and if the hosts supports it.
Only working if he user has already clicked on a knob in the plugin is the point -- that control has the focus, and can be nudged with the arrow keys. Like in many graphics programs, you can nudge the currently selected object that way. Yes, it only works when the IS a currently selected object, but that's how it works, it's an operation you can do on the selected object. In this context, it's the selected control, not an object.
One difference that may be an issue is that in graphics apps, there's always a visual cue about which object is selected, and as it stands, there's no indication which control has the focus, other than something in the host if it has one (Studio One does, but not on the control itself). A further possible improvement might be to add a gentle border or background color to indicate that, but to make this proposal as easy to implement as possible, unless that's super easy, I'd say try it without, see if users like it better than not having nudge capability at all.
As far as it not working in some hosts, my suggestion would be:
- Check out whether it works in most hosts, or only a few
- Abandon the idea if only a few can support it
- If most can, document that it only works of your host supports it, maybe document the ones that do
As long as you explain that it's NOT a bug (at least not in the plugin), it's just How It Is, I think you're in the clear, and you can offer this useful feature when it's possible. It's not a bug that the sidechain features of some plugins don't work in some hosts -- it depends whether both plugin and host support VST3, which has real sidechain support, and if not, whether the host's fake VST2 sidechain stuff works with a particular plugin. I think. Anyway, devs don't remove those features because they can't be supported on all hosts, they make them available when they can.
Speaking of host-dependent features, in Studio One you can type in a value for the selected control, but as I've said, in Saurus, when you press Enter to apply that value, it reverts to the current value of the control, doesn't update it. Is that something you have control over, or is it purely the host's problem?
Thanks again for listening