cmd_output: Merge instead of replace output config

include/config.h

@@ -99,6 +99,7 @@ bool read_config(FILE *file, bool is_active);
  */
 char *do_var_replacement(char *str);
 int output_name_cmp(const void *item, const void *data);
+void merge_output_config(struct output_config *dst, struct output_config *src);
 /** Sets up a WLC output handle based on a given output_config.
  */
 void apply_output_config(struct output_config *oc, swayc_t *output);

sway/commands.c

@@ -804,12 +804,14 @@ static struct cmd_results *cmd_output(int argc, char **argv) {
 
 	i = list_seq_find(config->output_configs, output_name_cmp, name);
 	if (i >= 0) {
-		// replace existing config
+		// merge existing config
 		struct output_config *oc = config->output_configs->items[i];
-		list_del(config->output_configs, i);
-		free_output_config(oc);
+		merge_output_config(oc, output);
+		free_output_config(output);
+		output = oc;
+	} else {
+		list_add(config->output_configs, output);
 	}
-	list_add(config->output_configs, output);
 
 	sway_log(L_DEBUG, "Config stored for output %s (enabled:%d) (%d x %d @ %d, %d) (bg %s %s)",
 			output->name, output->enabled, output->width,

sway/config.c

@@ -271,6 +271,42 @@ int output_name_cmp(const void *item, const void *data) {
 	return strcmp(output->name, name);
 }
 
+void merge_output_config(struct output_config *dst, struct output_config *src) {
+	if (src->name) {
+		if (dst->name) {
+			free(dst->name);
+		}
+		dst->name = strdup(src->name);
+	}
+	if (src->enabled != -1) {
+		dst->enabled = src->enabled;
+	}
+	if (src->width != -1) {
+		dst->width = src->width;
+	}
+	if (src->height != -1) {
+		dst->height = src->height;
+	}
+	if (src->x != -1) {
+		dst->x = src->x;
+	}
+	if (src->y != -1) {
+		dst->y = src->y;
+	}
+	if (src->background) {
+		if (dst->background) {
+			free(dst->background);
+		}
+		dst->background = strdup(src->background);
+	}
+	if (src->background_option) {
+		if (dst->background_option) {
+			free(dst->background_option);
+		}
+		dst->background_option = strdup(src->background_option);
+	}
+}
+
 void apply_output_config(struct output_config *oc, swayc_t *output) {
 	if (oc && oc->width > 0 && oc->height > 0) {
 		output->width = oc->width;