seat: support low-resolution clients

When the client doesn't support high-resolution scroll, accumulate
deltas until we can notify a discrete event.

Some mice have a free spinning wheel, making possible to lock the wheel
when the accumulator value is not 0. To avoid synchronization issues
between the mouse wheel and the accumulators, store the last delta and
when the scroll direction changes, reset the accumulator.

include/wlr/types/wlr_seat.h

@@ -54,6 +54,19 @@ struct wlr_seat_client {
 	// for use by wlr_seat_client_{next_serial,validate_event_serial}
 	struct wlr_serial_ringset serials;
 	bool needs_touch_frame;
+
+	// When the client doesn't support high-resolution scroll, accumulate deltas
+	// until we can notify a discrete event.
+	// Some mice have a free spinning wheel, making possible to lock the wheel
+	// when the accumulator value is not 0. To avoid synchronization issues
+	// between the mouse wheel and the accumulators, store the last delta and
+	// when the scroll direction changes, reset the accumulator.
+	// Indexed by wlr_axis_orientation.
+	struct {
+		int32_t acc_discrete[2];
+		int32_t last_discrete[2];
+		double acc_axis[2];
+	} value120;
 };
 
 struct wlr_touch_point {

types/seat/wlr_seat_pointer.c

@@ -269,6 +269,48 @@ uint32_t wlr_seat_pointer_send_button(struct wlr_seat *wlr_seat, uint32_t time,
 	return serial;
 }
 
+static bool should_reset_value120_accumulators(int32_t current, int32_t last) {
+	if (last == 0) {
+		return true;
+	}
+
+	return (current < 0 && last > 0) || (current > 0 && last < 0);
+}
+
+static void update_value120_accumulators(struct wlr_seat_client *client,
+		enum wlr_axis_orientation orientation,
+		double value, int32_t value_discrete) {
+	int32_t *acc_discrete = &client->value120.acc_discrete[orientation];
+	int32_t *last_discrete = &client->value120.last_discrete[orientation];
+	double *acc_axis = &client->value120.acc_axis[orientation];
+	if (should_reset_value120_accumulators(value_discrete, *last_discrete)) {
+		*acc_discrete = 0;
+		*acc_axis = 0;
+	}
+	*acc_discrete += value_discrete;
+	*last_discrete = value_discrete;
+	*acc_axis += value;
+}
+
+static void send_axis_discrete(struct wlr_seat_client *client,
+		struct wl_resource *resource, uint32_t time,
+		enum wlr_axis_orientation orientation, double value,
+		int32_t value_discrete) {
+	int32_t *acc_discrete = &client->value120.acc_discrete[orientation];
+	double *acc_axis = &client->value120.acc_axis[orientation];
+
+	if (abs(*acc_discrete) < WLR_POINTER_AXIS_DISCRETE_STEP) {
+		return;
+	}
+
+	wl_pointer_send_axis_discrete(resource, orientation,
+		*acc_discrete / WLR_POINTER_AXIS_DISCRETE_STEP);
+	wl_pointer_send_axis(resource, time, orientation,
+		wl_fixed_from_double(*acc_axis));
+	*acc_discrete %= WLR_POINTER_AXIS_DISCRETE_STEP;
+	*acc_axis = 0;
+}
+
 static void send_axis_value120(struct wl_resource *resource, uint32_t time,
 		enum wlr_axis_orientation orientation, double value,
 		int32_t value_discrete) {
@@ -294,6 +336,8 @@ void wlr_seat_pointer_send_axis(struct wlr_seat *wlr_seat, uint32_t time,
 		send_source = true;
 	}
 
+	update_value120_accumulators(client, orientation, value, value_discrete);
+
 	struct wl_resource *resource;
 	wl_resource_for_each(resource, &client->pointers) {
 		if (wlr_seat_client_from_pointer_resource(resource) == NULL) {
@@ -311,8 +355,8 @@ void wlr_seat_pointer_send_axis(struct wlr_seat *wlr_seat, uint32_t time,
 					send_axis_value120(resource, time, orientation, value,
 						value_discrete);
 				} else if (version >= WL_POINTER_AXIS_DISCRETE_SINCE_VERSION) {
-					wl_pointer_send_axis_discrete(resource, orientation,
-					value_discrete / WLR_POINTER_AXIS_DISCRETE_STEP);
+					send_axis_discrete(client, resource, time, orientation,
+						value, value_discrete);
 				}
 			} else {
 				wl_pointer_send_axis(resource, time, orientation,