xwayland: kinetic scroll doesn't work because stop events are not emitted

Applications running under XWayland don't receive scroll stop events. As a result, kinetic scroll doesn't work under XWayland.

The following hack can fix that. The Wayland server sends 2 scroll stop events when a toucpad scroll gesture ends. Translate that to a single X11 'smooth scroll' event with dx and dy both set to 0.

This is just a hack, maybe someone knowledegable can convert it to a proper fix.

diff --git a/hw/xwayland/xwayland-input.c b/hw/xwayland/xwayland-input.c
index 9c574314d..fc214f2ef 100644
--- a/hw/xwayland/xwayland-input.c
+++ b/hw/xwayland/xwayland-input.c
@@ -629,6 +629,24 @@ static void
 pointer_handle_axis_stop(void *data, struct wl_pointer *wl_pointer,
                          uint32_t time, uint32_t axis)
 {
+    struct xwl_seat *xwl_seat = data;
+    ValuatorMask mask;
+
+    /* temporary hack */
+    static int toggle = 0;
+
+    toggle = 1 - toggle;
+
+    /* pointer_handle_axis_stop() is called once for each axis, however X sends only
+       a single event for both axes */
+    if (toggle)
+        return;
+
+    valuator_mask_zero(&mask);
+    valuator_mask_set_double(&mask, 3, 0);
+    valuator_mask_set_double(&mask, 2, 0);
+
+    QueuePointerEvents(xwl_seat->pointer, MotionNotify, 0, POINTER_RELATIVE, &mask);
 }
 
 static void