ipc-server: Force modeset if needed after executing commands

IPC clients generally expect executed commands to have taken effect when
the command completes, while delayed modeset means that it can take
several milliseconds more before e.g. an output is enabled.

However, modesetting on every output command in the IPC call could on
systems with already slow modesetting behavior lead to an unresponsive
system for a not insignificant period of time.

To strike a balance, force modeset once all the commands of this IPC
call have executed if a modeset is pending.

include/sway/config.h

@@ -706,6 +706,7 @@ void free_output_config(struct output_config *oc);
 
 void request_modeset(void);
 void force_modeset(void);
+bool modeset_is_pending(void);
 
 bool spawn_swaybg(void);
 

sway/desktop/output.c

@@ -408,6 +408,10 @@ void request_modeset(void) {
 	}
 }
 
+bool modeset_is_pending(void) {
+	return server.delayed_modeset != NULL;
+}
+
 void force_modeset(void) {
 	if (server.delayed_modeset != NULL) {
 		wl_event_source_remove(server.delayed_modeset);

sway/ipc-server.c

@@ -648,6 +648,12 @@ void ipc_client_handle_command(struct ipc_client *client, uint32_t payload_lengt
 		}
 
 		list_t *res_list = execute_command(buf, NULL, NULL);
+		if (modeset_is_pending()) {
+			// IPC expects commands to have taken immediate effect, so we need
+			// to force a modeset after output commands. We do a single modeset
+			// here to avoid modesetting for every output command in sequence.
+			force_modeset();
+		}
 		transaction_commit_dirty();
 		char *json = cmd_results_to_json(res_list);
 		int length = strlen(json);