Fix darkroom scroll zooming on macOS using magic mouse and pinch zooming on 2nd window

[da-phil]

This PR tries to fix the following regressions caused in #20812:

• zooming via scroll gestures on macOS using the magic mouse stopped working (see comment RFC: How should darkroom zooming-in behave? #21027 (comment))
• pinch zooming on 2nd window stopped working (see comment RFC: How should darkroom zooming-in behave? #21027 (comment))

[da-phil]

@MStraeten can you please give this fix a try and share the logs? Thanks.
Does scroll zoom work while holding CTRL?

[MStraeten]

zooming by dragging on magic mouse and trackpad is fine - unless pinch and zoom was done on trackpad.
After that zooming in by mouse / trackpad is just possible in combination with control key. In a zoomed in view draging scrolls the view.

log.txt

[da-phil]

zooming by dragging on magic mouse and trackpad is fine - unless pinch and zoom was done on trackpad. After that zooming in by mouse / trackpad is just possible in combination with control key. In a zoomed in view draging scrolls the view.

log.txt

Ah yes, that's also the behavior on Linux with a mouse & touchpad, given that we cannot resolve the ambiguity between scroll-zooming (linearly moving both fingers up/down in parallel) and panning gestures (moving both fingers in either direction), without using another key to differentiate them. As far as I remember, dragging (moving mouse / single finger while clicking) has always been doing a panning motion.
I thought that the Apple Magic Mouse was doing something special here, it seems that it works the same as on Linux.

Then we come to the design decision which was

• requested in this feature request: Add pinch to zoom in and out for trackpads/touchpads #18048
• implemented in those PRs: add touchpad zoom and pan gestures #20356 & re-enable touchpad gestures with regression fixes #20451

The rationale:

Adds touchpad gestures in Darkroom:

pinch zoom in/out
two-finger pan

After above PRs were merged, two finger gestures (including the scroll one) became the panning gesture, just like the usual dragging works (holding left click / touchpad tap and then moving). And zooming is done with the dedicated and universally used pinch-zoom gesture.
At least I could verify locally on my Linux setup when going back before #20451 that this behavior changed.

Does reverting my PR / going back to a commit before my PR merged (e.g. d436623) bring back the behavior you expect?
This would be very surprising for me.

[da-phil]

zooming by dragging on magic mouse and trackpad is fine - unless pinch and zoom was done on trackpad. After that zooming in by mouse / trackpad is just possible in combination with control key. In a zoomed in view draging scrolls the view.

log.txt

By the way, thanks for the log file, this made the following clear:
The failure isn't "broken zoom" in general - it's that on macOS, Magic Mouse and trackpad share a GdkDevice instance, and after the trackpad pinch binds _touchpad to that shared device, every Magic Mouse scroll satisfies matches_last_gesture_device and gets routed to pan. Now clearing _touchpad on pinch END/CANCEL fixes the Magic Mouse case but as your description and log shows, it would re-break the design intent that trackpad two-finger drag should pan even without a prior pinch. The two fixes are mutually exclusive as long as we rely on device identity to discriminate them.

I need to have another look if I find more time later.