seat/keyboard: support sending modifiers when not focused

This adds wlr_seat_client_notify_modifiers which sends the current
keyboard modifiers to an additional client, tracking what the client
last saw to avoid unneeded duplicates. This is useful to make actions
like "ctrl+scroll" work correctly when pointer and keyboard focus are on
distinct clients.

The tracked modifier state is also used to avoid sending unneeded
modifiers events in default_keyboard_modifiers.  No de-duplication is
performed in wlr_seat_keyboard_send_modifiers because that function is
used in cases where the protocol unconditionally requires a modifiers
event to be sent (such as after wl_keyboard_enter).

include/wlr/types/wlr_seat.h

@@ -55,6 +55,13 @@ struct wlr_seat_client {
 	struct wlr_serial_ringset serials;
 	bool needs_touch_frame;
 
+	// The client's current knowledge of the keyboard modifier state.  This
+	// will lag behind the actual state if the client does not have
+	// keyboard focus; it is used by wlr_seat_client_notify_modifiers to
+	// allow actions like ctrl+scroll to work without sending duplicate
+	// events if modifiers are unchanged.
+	struct wlr_keyboard_modifiers tracked_modifiers;
+
 	// 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
@@ -526,6 +533,14 @@ void wlr_seat_keyboard_send_key(struct wlr_seat *seat, uint32_t time_msec,
 void wlr_seat_keyboard_send_modifiers(struct wlr_seat *seat,
 		const struct wlr_keyboard_modifiers *modifiers);
 
+/**
+ * Update an additional client's view of the keyboard modifier state.  This
+ * does nothing if the target has keyboard focus or if its view of the modifier
+ * state still matches the current modifier state.
+ */
+void wlr_seat_client_notify_modifiers(struct wlr_seat *seat,
+		struct wlr_seat_client *target);
+
 /**
  * Send a keyboard enter event to the given surface and consider it to be the
  * focused surface for the keyboard. This will send a leave event to the last

types/seat/wlr_seat_keyboard.c

@@ -10,6 +10,11 @@
 #include "types/wlr_data_device.h"
 #include "types/wlr_seat.h"
 
+static void seat_keyboard_send_modifiers(struct wlr_seat *seat,
+		const struct wlr_keyboard_modifiers *modifiers,
+		struct wlr_seat_client *client,
+		bool force);
+
 static void default_keyboard_enter(struct wlr_seat_keyboard_grab *grab,
 		struct wlr_surface *surface, const uint32_t keycodes[], size_t num_keycodes,
 		const struct wlr_keyboard_modifiers *modifiers) {
@@ -27,7 +32,7 @@ static void default_keyboard_key(struct wlr_seat_keyboard_grab *grab,
 
 static void default_keyboard_modifiers(struct wlr_seat_keyboard_grab *grab,
 		const struct wlr_keyboard_modifiers *modifiers) {
-	wlr_seat_keyboard_send_modifiers(grab->seat, modifiers);
+	seat_keyboard_send_modifiers(grab->seat, modifiers, NULL, false);
 }
 
 static void default_keyboard_cancel(struct wlr_seat_keyboard_grab *grab) {
@@ -195,34 +200,67 @@ static void seat_keyboard_handle_surface_destroy(struct wl_listener *listener,
 	wlr_seat_keyboard_clear_focus(state->seat);
 }
 
-void wlr_seat_keyboard_send_modifiers(struct wlr_seat *seat,
+static void seat_keyboard_send_modifiers(struct wlr_seat *seat,
-		const struct wlr_keyboard_modifiers *modifiers) {
+		const struct wlr_keyboard_modifiers *modifiers,
-	struct wlr_seat_client *client = seat->keyboard_state.focused_client;
+		struct wlr_seat_client *client,
+		bool force) {
+	const struct wlr_keyboard_modifiers empty = {};
+	struct wl_resource *resource;
+
 	if (client == NULL) {
+		client = seat->keyboard_state.focused_client;
+	}
+	if (client == NULL) {
+		return;
+	}
+	if (modifiers == NULL) {
+		modifiers = ∅
+	}
+
+	struct wlr_keyboard_modifiers *tracked = &client->tracked_modifiers;
+	if (!force && tracked->depressed == modifiers->depressed &&
+			tracked->latched == modifiers->latched &&
+			tracked->locked == modifiers->locked &&
+			tracked->group == modifiers->group) {
 		return;
 	}
 
 	uint32_t serial = wlr_seat_client_next_serial(client);
-	struct wl_resource *resource;
+	*tracked = *modifiers;
+
 	wl_resource_for_each(resource, &client->keyboards) {
 		if (seat_client_from_keyboard_resource(resource) == NULL) {
 			continue;
 		}
 
-		if (modifiers == NULL) {
+		wl_keyboard_send_modifiers(resource, serial,
-			wl_keyboard_send_modifiers(resource, serial, 0, 0, 0, 0);
+			modifiers->depressed, modifiers->latched,
-		} else {
+			modifiers->locked, modifiers->group);
-			wl_keyboard_send_modifiers(resource, serial,
-				modifiers->depressed, modifiers->latched,
-				modifiers->locked, modifiers->group);
-		}
 	}
 }
 
+void wlr_seat_keyboard_send_modifiers(struct wlr_seat *seat,
+		const struct wlr_keyboard_modifiers *modifiers) {
+	seat_keyboard_send_modifiers(seat, modifiers, NULL, true);
+}
+
+void wlr_seat_client_notify_modifiers(struct wlr_seat *seat,
+		struct wlr_seat_client *target) {
+	struct wlr_seat_client *focused = seat->keyboard_state.focused_client;
+	if (target == NULL || target == focused) {
+		return;
+	}
+
+	struct wlr_keyboard_modifiers *modifiers = &seat->keyboard_state.keyboard->modifiers;
+	seat_keyboard_send_modifiers(seat, modifiers, target, false);
+}
+
 void seat_client_send_keyboard_leave_raw(struct wlr_seat_client *seat_client,
 		struct wlr_surface *surface) {
 	uint32_t serial = wlr_seat_client_next_serial(seat_client);
 	struct wl_resource *resource;
+	const struct wlr_keyboard_modifiers unset = {};
+	seat_client->tracked_modifiers = unset;
 	wl_resource_for_each(resource, &seat_client->keyboards) {
 		if (seat_client_from_keyboard_resource(resource) == NULL) {
 			continue;