evdev: Wait for SYN event before sending events over to the client

The issue was that touch::down event from the compositor to client apps
would send the previous motion events coordinates and this obviously made
the client do the wrong thing. This happened because we were not waiting
for a SYN event to come from evdev before sending down, motion or up events.

https://bugs.freedesktop.org/show_bug.cgi?id=51909

src/evdev-touchpad.c

@@ -481,7 +481,7 @@ touchpad_update_state(struct touchpad_dispatch *touchpad, uint32_t time)
 			touchpad->device->rel.dx = wl_fixed_from_double(dx);
 			touchpad->device->rel.dy = wl_fixed_from_double(dy);
 			touchpad->device->pending_events |=
-				EVDEV_RELATIVE_MOTION;
+				EVDEV_RELATIVE_MOTION | EVDEV_SYN;
 		} else if (touchpad->finger_state == TOUCHPAD_FINGERS_TWO) {
 			if (dx != 0.0)
 				notify_axis(touchpad->device->seat,

src/evdev.c

@@ -249,9 +249,10 @@ evdev_flush_motion(struct evdev_device *device, uint32_t time)
 {
 	struct weston_seat *master = device->seat;
 
-	if (!device->pending_events)
+	if (!(device->pending_events & EVDEV_SYN))
 		return;
 
+	device->pending_events &= ~EVDEV_SYN;
 	if (device->pending_events & EVDEV_RELATIVE_MOTION) {
 		notify_motion(master, time,
 			      master->seat.pointer->x + device->rel.dx,
@@ -308,6 +309,9 @@ fallback_process(struct evdev_dispatch *dispatch,
 	case EV_KEY:
 		evdev_process_key(device, event, time);
 		break;
+	case EV_SYN:
+		device->pending_events |= EVDEV_SYN;
+		break;
 	}
 }
 

src/evdev.h

@@ -34,6 +34,7 @@ enum evdev_event_type {
 	EVDEV_ABSOLUTE_MT_MOTION = (1 << 2),
 	EVDEV_ABSOLUTE_MT_UP = (1 << 3),
 	EVDEV_RELATIVE_MOTION = (1 << 4),
+	EVDEV_SYN = (1 << 5),
 };
 
 enum evdev_device_capability {