Preserving previous output

include/stringop.h

@@ -3,6 +3,7 @@
 
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdarg.h>
 #include "list.h"
 
 #ifdef __GNUC__

include/sway/output.h

@@ -39,6 +39,8 @@ struct sway_output {
 	struct wlr_scene_rect *fullscreen_background;
 
 	struct wlr_output *wlr_output;
+	char *previous_output_name;
+
 	struct wlr_scene_output *scene_output;
 	struct sway_server *server;
 	struct wl_list link;

include/sway/tree/workspace.h

@@ -45,6 +45,7 @@ struct sway_workspace {
 	struct side_gaps gaps_outer;
 
 	struct sway_output *output; // NULL if no outputs are connected
+	char *previous_output_name; // NULL if no previous output
 	list_t *floating;           // struct sway_container
 	list_t *tiling;             // struct sway_container
 	list_t *output_priority;