wayland: handle axis_value120 scroll events

This can be tested with the `application` example, looking at the events
shown for mouse wheel movement.

`wl_pointer::axis_discrete` isn't sent in version 8 or higher of
`wl_pointer`. And `sctk` doesn't convert the `value120` events, so
on compositors advertising version 8, only pixel scroll events were
being sent.

This sends `MouseScrollDelta::LineDelta` with a fractional value,
without doing any accumulation. Given `LineDelta` contains `f32` values,
this presumably is expected?

Though it might be good to change the definition of `MouseScrollDelta`
to include both discrete and pixel values, when the compositor sends
both. I'm not familiar with how this works on non-Wayland backends
though.

winit-wayland/src/seat/pointer/mod.rs

@@ -195,6 +195,7 @@ impl PointerHandler for WinitState {
                     // Get the current phase.
                     let mut pointer_data = pointer.winit_data().inner.lock().unwrap();
 
+                    let has_value120_scroll = horizontal.value120 != 0 || vertical.value120 != 0;
                     let has_discrete_scroll = horizontal.discrete != 0 || vertical.discrete != 0;
 
                     // Figure out what to do about start/ended phases here.
@@ -206,7 +207,7 @@ impl PointerHandler for WinitState {
                     } else {
                         match pointer_data.phase {
                             // Discrete scroll only results in moved events.
-                            _ if has_discrete_scroll => TouchPhase::Moved,
+                            _ if has_value120_scroll || has_discrete_scroll => TouchPhase::Moved,
                             TouchPhase::Started | TouchPhase::Moved => TouchPhase::Moved,
                             _ => TouchPhase::Started,
                         }
@@ -217,7 +218,13 @@ impl PointerHandler for WinitState {
 
                     // Mice events have both pixel and discrete delta's at the same time. So prefer
                     // the discrete values if they are present.
-                    let delta = if has_discrete_scroll {
+                    let delta = if has_value120_scroll {
+                        // NOTE: Wayland sign convention is the inverse of winit.
+                        MouseScrollDelta::LineDelta(
+                            (-horizontal.value120) as f32 / 120.,
+                            (-vertical.value120) as f32 / 120.,
+                        )
+                    } else if has_discrete_scroll {
                         // NOTE: Wayland sign convention is the inverse of winit.
                         MouseScrollDelta::LineDelta(
                             (-horizontal.discrete) as f32,

winit/src/changelog/unreleased.md

@@ -268,3 +268,4 @@ changelog entry.
 - On Windows, `Window::set_theme` will change the title bar color immediately now.
 - On Windows 11, prevent incorrect shifting when dragging window onto a monitor with different DPI.
 - On Web, device events are emitted regardless of cursor type.
+- On Wayland, `axis_value120` scroll events now generate `MouseScrollDelta::LineDelta`