Touchpad Scroll on Wayland not working

[jwrober]

Describe the bug
The touchpad scroll only works left/right, up/down and not diagonally on Wayland. Is also laggy/slow. Works fine on X11

To Reproduce
Steps to reproduce the behavior:

1. Set your Linux to use Wayland instead of X11
2. Open a game
3. See error

Expected behavior
Mouse events work same on Wayland as on X11

Screenshots
N/A

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: 3.1-alpha.5
• Ruleset/Longturn game (if applicable): N/A

Additional context
https://discord.com/channels/378908274113904641/1109113177779732550/1215113440033636464

[lmoureaux]

Is this KWin-specific or do you see the same on Mutter?

[jwrober]

I'm also on KWin and can reproduce there.

[lmoureaux]

But is it related to KWin or does Gnome give the same?

[blabber]

FWIW, I can't reproduce this on FreeBSD running sway

[lmoureaux]

Works like a charm when using the xcb backend with QT_QPA_PLATFORM=xcb (so going through XWayland).

[lmoureaux]

Scrolling events come in two modes, pixel-based and angle-based. On we get deltas of the form pixel=QPoint(0,0) angle=QPoint(0,21) and use the angle ones. On wayland we get deltas like pixel=QPoint(0,-4) angle=QPoint(0,-58), and use the pixel ones.
From the documentation:

Wheel events are generated for both mouse wheels and trackpad scroll gestures. There are two ways to read the wheel event delta: angleDelta() returns the deltas in wheel degrees. These values are always provided. pixelDelta() returns the deltas in screen pixels, and is available on platforms that have high-resolution trackpads, such as macOS. If that is the case, source() will return Qt::MouseEventSynthesizedBySystem.

Events on Wayland have Qt::MouseEventSynthesizedBySystem, events on XCB have Qt::MouseEventNotSynthesized.

--> How are we supposed to use the pixel deltas in the first place? They don't feel like pixel differences at all.

[lmoureaux]

--> How are we supposed to use the pixel deltas in the first place? They don't feel like pixel differences at all.

Qt itself uses angleDelta everywhere. Will follow suit