Linux: Inversed MouseScrollDelta::LineDelta values

[alexheretic]

Since updating winit 0.6 -> 0.7 I've noticed the MouseScrollDelta::LineDelta(_, y) y values have inverted on Linux x11 & wayland. So now the behaviour is opposite of the documentation. The previous/correct behaviour is 1 scrolling away from yourself, -1 scrolling back.

Testing the same case on windows, and the y value was correct as documented. I didn't test on mac, but I'm presuming it's a Linux regression. Can anyone else confirm this on their setup?

[Ralith]

Discussion in #218 may be of interest.

The previous/correct behaviour is 1 scrolling away from yourself, -1 scrolling back.

Reviewing the documentation on the enum, it looks like I just got this wrong when I rewrote the X11 backend. Which makes it confusing that you observe the same behavior in Wayland, which afaik did not undergo major changes in that period.

[jwilm]

Which makes it confusing that you observe the same behavior in Wayland, which afaik did not undergo major changes in that period.

It's possible the X11 backend was still being used under @alexheretic's testing.

[alexheretic]

I can confirm the incorrect behaviour in both x11 & wayland backends. Running a wayland session, as I do, the unix backend can be chosen using WINIT_UNIX_BACKEND. Also worth noting that currently the two backend's appearance cannot be mistaken; wayland is half transparent and has minimal window decoration.

[Ralith]

Maybe @vberger has some insight into what's going on there?

[alexheretic]

To be totally accurate, I know this worked in 0.6 in x11. And now I can test it not working in both x & wayland.
It is possible it may not have been correct in wayland before. (My older code doesn't work at all with a 0.6 wayland backend, so I can't test).

[elinorbgr]

Currently the wayland backend simply aggregates and forwards scrolling values it got from the compositor. Apparently, the convention for wayland on this is the opposite of the one chosen in winit. This is entirely possible that this was wrong from the start. 😅

It should be as simple a fix as flipping the signs here : https://github.com/tomaka/winit/blob/master/src/platform/linux/wayland/event_loop.rs#L458-L459

However, this raises the question of horizontal scrolling, are the convention mixed here as well? It looks like the convention on wayland is that the signs of the scroll delta are positive when going down or right, thus matching the conventions of the mouse deltas when moving the pointer.

[Ralith]

MouseScrollDelta::LineDelta says:

Positive values indicate movement forward (away from the user) or rightwards.

So, horizontal scrolling should be fine.