input/pointer: send axis source once per frame

Only one wl_pointer.axis_source event is permitted per frame, according
to the Wayland specification.

Fixes https://github.com/swaywm/wlroots/issues/2973

include/wlr/types/wlr_seat.h

@@ -164,6 +164,9 @@ struct wlr_seat_pointer_state {
 	struct wlr_seat_pointer_grab *grab;
 	struct wlr_seat_pointer_grab *default_grab;
 
+	bool sent_axis_source;
+	enum wlr_axis_source cached_axis_source;
+
 	uint32_t buttons[WLR_POINTER_BUTTONS_CAP];
 	size_t button_count;
 	uint32_t grab_button;

types/seat/wlr_seat_pointer.c

@@ -274,6 +274,15 @@ void wlr_seat_pointer_send_axis(struct wlr_seat *wlr_seat, uint32_t time,
 		return;
 	}
 
+	bool send_source = false;
+	if (wlr_seat->pointer_state.sent_axis_source) {
+		assert(wlr_seat->pointer_state.cached_axis_source == source);
+	} else {
+		wlr_seat->pointer_state.sent_axis_source = true;
+		wlr_seat->pointer_state.cached_axis_source = source;
+		send_source = true;
+	}
+
 	struct wl_resource *resource;
 	wl_resource_for_each(resource, &client->pointers) {
 		if (wlr_seat_client_from_pointer_resource(resource) == NULL) {
@@ -282,7 +291,7 @@ void wlr_seat_pointer_send_axis(struct wlr_seat *wlr_seat, uint32_t time,
 
 		uint32_t version = wl_resource_get_version(resource);
 
-		if (version >= WL_POINTER_AXIS_SOURCE_SINCE_VERSION) {
+		if (send_source && version >= WL_POINTER_AXIS_SOURCE_SINCE_VERSION) {
 			wl_pointer_send_axis_source(resource, source);
 		}
 		if (value) {
@@ -306,6 +315,8 @@ void wlr_seat_pointer_send_frame(struct wlr_seat *wlr_seat) {
 		return;
 	}
 
+	wlr_seat->pointer_state.sent_axis_source = false;
+
 	struct wl_resource *resource;
 	wl_resource_for_each(resource, &client->pointers) {
 		if (wlr_seat_client_from_pointer_resource(resource) == NULL) {