backend/wayland: support touch cancel events

since wayland doesn't provide a touch id in cancel events, track what
points are active so we can cancel all of them

timestamp is also not provided - use 0 because no one's paying attention
to that anyway

Closes #3000

backend/wayland/seat.c

@@ -141,6 +141,10 @@ static void touch_handle_down(void *data, struct wl_touch *wl_touch,
 	struct wlr_wl_seat *seat = data;
 	struct wlr_touch *touch = &seat->wlr_touch;
 
+	struct wlr_wl_touch_points *points = &seat->touch_points;
+	assert(points->len != sizeof(points->ids) / sizeof(points->ids[0]));
+	points->ids[points->len++] = id;
+
 	struct wlr_touch_down_event event = {
 		.touch = touch,
 		.time_msec = time,
@@ -150,11 +154,26 @@ static void touch_handle_down(void *data, struct wl_touch *wl_touch,
 	wl_signal_emit_mutable(&touch->events.down, &event);
 }
 
+static bool remove_touch_point(struct wlr_wl_touch_points *points, int32_t id) {
+	size_t i = 0;
+	for (; i < points->len; i++) {
+		if (points->ids[i] == id) {
+			size_t remaining = points->len - i - 1;
+			memmove(&points->ids[i], &points->ids[i + 1], remaining * sizeof(id));
+			points->len--;
+			return true;
+		}
+	}
+	return false;
+}
+
 static void touch_handle_up(void *data, struct wl_touch *wl_touch,
 		uint32_t serial, uint32_t time, int32_t id) {
 	struct wlr_wl_seat *seat = data;
 	struct wlr_touch *touch = &seat->wlr_touch;
 
+	remove_touch_point(&seat->touch_points, id);
+
 	struct wlr_touch_up_event event = {
 		.touch = touch,
 		.time_msec = time,
@@ -184,7 +203,19 @@ static void touch_handle_frame(void *data, struct wl_touch *wl_touch) {
 }
 
 static void touch_handle_cancel(void *data, struct wl_touch *wl_touch) {
-	// no-op
+	struct wlr_wl_seat *seat = data;
+	struct wlr_touch *touch = &seat->wlr_touch;
+
+	// wayland's cancel event applies to all active touch points
+	for (size_t i = 0; i < seat->touch_points.len; i++) {
+		struct wlr_touch_cancel_event event = {
+			.touch = touch,
+			.time_msec = 0,
+			.touch_id = seat->touch_points.ids[i],
+		};
+		wl_signal_emit_mutable(&touch->events.cancel, &event);
+	}
+	seat->touch_points.len = 0;
 }
 
 static void touch_handle_shape(void *data, struct wl_touch *wl_touch,

include/backend/wayland.h

@@ -110,6 +110,11 @@ struct wlr_wl_pointer {
 	struct wl_list link;
 };
 
+struct wlr_wl_touch_points {
+	int32_t ids[64];
+	size_t len;
+};
+
 struct wlr_wl_seat {
 	char *name;
 	struct wl_seat *wl_seat;
@@ -131,6 +136,7 @@ struct wlr_wl_seat {
 
 	struct wl_touch *wl_touch;
 	struct wlr_touch wlr_touch;
+	struct wlr_wl_touch_points touch_points;
 
 	struct zwp_tablet_seat_v2 *zwp_tablet_seat_v2;
 	struct zwp_tablet_v2 *zwp_tablet_v2;