Native: Scroll direction varies by platform

[EmbersArc]

Describe the bug
As we noticed in #349 (comment):
On Linux, the two-finger scroll delta reported by ui.input().scroll_delta is positive to the right and up. I can enable "natural scrolling" in the settings and this reverses the direction on both axes. Apparently this is different on other platforms, namely on Mac.

To Reproduce
Steps to reproduce the behavior:

1. Use ui.input().scroll_delta to get the scroll delta when using the touchpad with two fingers
2. The signs of the values may vary depending on the platform that egui is used on

Expected behavior
That the reported values are always the same.

Desktop:
Operating System: Manjaro Linux
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.81.0
Qt Version: 5.15.2
Kernel Version: 5.12.1-2-MANJARO
OS Type: 64-bit
Graphics Platform: X11
Processors: 12 × Intel® Core™ i7-9750H CPU @ 2.60GHz
Memory: 15.3 GiB of RAM
Graphics Processor: GeForce GTX 1650 with Max-Q Design/PCIe/SSE2

[emilk]

On my macbook (macOS Mojave) the positive scroll direction is down and to the right (with "natural scrolling" turned on). This is the correct direction, and same as the positive mouse delta.

[emilk]

I'd like to get a few more voices on this before taking action. This may be an issue with KDE, Linux, or all non-Mac platforms.

So please test the scroll direction by checking out latest master, cargo run --release and check the scroll_delta values in the "Inspection" window.

[follower]

It might be helpful to re-title this issue "...scroll delta sign/direction..." given that there is was also a recently existing issue (#159) which is actually about the magnitude of the scroll value.

[akhilman]

I tested two finger scrolling on Xfce-4.12 with "Reverse scroll direction" enabled.
Moving fingers up and right gives negative values, left and down - positive values.
Overall the up/down scroll direction is correct (as in any other app). But the speed is low.

[emilk]

This should be worked on in winit if anyone has the energy to work on it

[aevyrie]

On windows, I'm experiencing horizontal scroll going the wrong direction.
Scrolling anywhere other than egui results in "natural" scrolling in x and y.
In egui (native), scrolling in x is flipped, y is correct. This is not the case on the web, where scrolling works as expected.

[akhilman]

Same on gnome/wayland. X-axis is flipped.

…

On Fri, 2021-12-17 at 10:31 -0800, Aevyrie wrote: On windows, I'm experiencing horizontal scroll going the wrong direction. Scrolling anywhere other than egui results in "natural" scrolling in x and y. In egui (native), scrolling in x is flipped, y is correct. This is not the case on the web, where scrolling works as expected.

[aevyrie]

I've PR'd winit. If anyone else has platforms with the same issue, take a look at my PR. It's super small change to the platform impl.

[emilk]

Fix wrong hscroll in winit:

• Reverse horizontal scroll direction rust-windowing/winit#2105