libsdl2 @2.0.10: trackpad working erratically as pointing device

[kencu (Ken)]

This may well turn out to be an upstream issue in the end, but I'm noticing it on our libsdl2 on 10.6.8 and I'm not sure how widespread it is just now.

After upgrading to libsdl2 @2.0.10, various ports that use libsdl2 have significant artifacts using the trackpad on my laptop (running 10.6.8/libc++).

The mousein / mouseout / mousedown events from a user perspective are quite unstable, almost unusable. Flashing rapidly, enabling on and off, while in the button zone, sometimes responding to clicks if the button happens to be active at the moment of the click, but usually not.

Interestingly, plugging in a USB mouse works perfectly normally again.

Also, reverting to libsdl2 @2.0.9 fixes the issues with the trackpad.

I see <​https://discourse.libsdl.org/t/sdl-2-0-10-released/26429> some work was done specifically on the trackpad and controller code in this update.

At this moment, I'm not sure if this is an issue with libsdl2 only on 10.6.8, or if it is the specific hardware on this laptop, or if it is general to @2.10.0 (it would seem hard to imagine this slipping past beta testers).

More investigation to follow. For now, workarounds are:

1. use a mouse
2. downgrade to libsdl2 @2.0.9

I'll assign this to Josh as it's his port, but we may find this issue belongs upstream rather than at this level if it's widespread.

[kencu (Ken)]

relevant <​https://hg.libsdl.org/SDL/rev/eb60e952b13f>