fixed logic for keeping track of touch points

include/slurp.h

@@ -9,6 +9,8 @@
 #include "wlr-layer-shell-unstable-v1-client-protocol.h"
 #include "xdg-output-unstable-v1-client-protocol.h"
 
+#define TOUCH_ID_EMPTY -1
+
 struct slurp_box {
 	int32_t x, y;
 	int32_t width, height;
@@ -82,8 +84,8 @@ struct slurp_seat {
 	int32_t anchor_x, anchor_y;
 	struct slurp_box selection;
 	bool has_selection;
-	bool already_touched;
-	
+	int32_t touch_id;
+
 	// pointer:
 	struct wl_pointer *wl_pointer;
 	enum wl_pointer_button_state button_state;

main.c

@@ -236,8 +236,8 @@ static void touch_handle_down(void *data, struct wl_touch *touch,
 		struct wl_surface *surface, int32_t id,
 		wl_fixed_t x, wl_fixed_t y) {
 	struct slurp_seat *seat = data;
-	if (!seat->already_touched) {
-		seat->already_touched = true;
+	if (seat->touch_id == TOUCH_ID_EMPTY) {
+		seat->touch_id = id;
 		seat->current_output =
 			output_from_surface(seat->state, surface);
 		move_seat(seat, x, y);
@@ -246,7 +246,7 @@ static void touch_handle_down(void *data, struct wl_touch *touch,
 }
 
 static void touch_clear_state(struct slurp_seat *seat) {
-	seat->already_touched = false;
+	seat->touch_id = TOUCH_ID_EMPTY;
 	seat->current_output = NULL;
 }
 
@@ -261,9 +261,11 @@ static void touch_handle_motion(void *data, struct wl_touch *touch,
 		uint32_t time, int32_t id, wl_fixed_t x,
 		wl_fixed_t y) {
 	struct slurp_seat *seat = data;
-	move_seat(seat, x, y);
-	handle_active_selection_motion(seat);
-	seat_set_outputs_dirty(seat);
+	if (seat->touch_id == id) {
+		move_seat(seat, x, y);
+		handle_active_selection_motion(seat);
+		seat_set_outputs_dirty(seat);
+	}
 }
 
 static void touch_handle_cancel(void *data, struct wl_touch *touch) {
@@ -311,6 +313,7 @@ static void create_seat(struct slurp_state *state, struct wl_seat *wl_seat) {
 	}
 	seat->state = state;
 	seat->wl_seat = wl_seat;
+	seat->touch_id = TOUCH_ID_EMPTY;
 	wl_list_insert(&state->seats, &seat->link);
 	wl_seat_add_listener(wl_seat, &seat_listener, seat);
 }