wrt feature detection:

on a Mac with a trackpad, you can enable and disable individual gestures in System Settings. If you've got a Magic Mouse, you can also map some of the gestures on the mouse surface to the system gestures. I kind of assumed that there must be some API to see if these gestures are enabled or not? But if there is I can't find it. Admittedly that doesn't mean much because Cocoa is a gigantic decades-old API and it very well could be in there somewhere.

I'm posting about it on the dev forums now, but I'm not sure it'll get anywhere.

This seems, at first glance, like a good approach to handle system-provided gestures.

For keyboard events, normally we use a ButtonThrower, rather than throwing directly at the MouseWatcher. Maybe we want a GestureThrower?

Do we need support in the GUI system? If so we need MouseWatcher to pass the gestures to the MouseWatcherRegion. At least for two-finger scroll--or is that not a gesture?

What do we do with pointer events? Are they suppressed when gestures are detected?

Checking other platforms is a good idea. I don't know about other platforms, but Android has GestureDetector:
https://developer.android.com/reference/android/view/GestureDetector

I suspect that on some platforms we might need to provide our own gesture detection. I suppose we could create a data graph node for that at some point (that takes in pointer events and emits gesture events and filtered pointer events). But that's out of scope here and this approach is still compatible with that.

I wasn't super sure if regions made sense for gestures, but on reflection I think so - gestures are recognised at the view level; they're sent to the view that the cursor is over when the gesture starts and keep getting sent to that view until the gesture ends. If you've got say, a minimap, you definitely want a zoom gesture in the minimap to zoom the minimap.

I made a little toy app to mess with this stuff, and I found some weird behaviour that I think is worth learning from. If you start a zoom gesture in one view, then move the cursor to a subview and start a rotate gesture, the subview now receives all gesture events, including magnify gesture events, and the original view never gets the gesture ended event. This feels like a bug? Even though it's very unlikely to get hit - you need to be using a mouse and a trackpad simultaneously - I think the sensible thing to say is that a gesture of a given type is locked to a specific region until it hits the ENDED or CANCELLED phase, just like we do with mouse clicks and _preferred_button_down_region. (If it has a BEGAN phase, that is… SWIPE events only have a CHANGED phase, no beginning or end.)

Re: GestureThrower – I was confused by the two totally separate code paths for handling button presses… I think I see now, one is for the GUI and one is for the scene. I'm not sure I understand why? I get that you don't want a key press that was consumed by the UI to be forwarded on to the scene, but I'm not sure what the point of ButtonThrower is when it seems like MouseWatcher implements pretty much the exact same functionality. If there's no region for it to throw to, can't it just drop the event parameter in throw_event_pattern? What am I missing here?

Re: Two finger scroll – at the moment this is handled as a MouseWheel button, and as far as I can tell is not counted as a "gesture" by macOS. NSPanGestureRecognizer isn't triggered by it. I think MouseWheel being a button is… not great? But that's a different issue.

I agree that when you start a gesture it should be locked to that region.

Yes, MouseWatcher is designed to filter out events that are meant for the GUI system. As for the separation between ButtonThrower and MouseWatcher - I am not privy to the rationale for the design decisions, but I imagine that the designers thought it was good to separate these duties, which perhaps allows for more flexibility such as obviating the need for MouseWatcher in a GUI-less application, creating multiple ButtonThrowers with different prefixes, or allowing other data graph configurations that I can't imagine. Note that there also isn't a MouseWatcher for other input devices such as gamepads (those directly get a ButtonThrower attached), but those don't generate gestures anyway. I don't know what the pattern events on MouseWatcher are used for.

We ought to fix the two-finger scroll being a wheel event. I think there ought to be a scroll event with delta values. I don't know if that should be considered a gesture or not (for macOS, these events probably predated the gesture system), but as you say, it's probably out of scope here.