make the Cage multi-monitor layout configurable

cage.1.scd

@@ -27,6 +27,11 @@ activities outside the scope of the running application are prevented.
 	*last* Cage uses only the last connected monitor.
 	*extend* Cage extends the display across all connected monitors.
 
+*-e* <mode>
+	Set the monitor addition mode, if display is extended across all connected monitors.
+	*auto* Cage places monitors automatically.
+	*right* Cage places each newly-connected monitor on the right of the rightmost connected monitor.
+
 *-r*
 	Rotate the output 90 degrees clockwise. This can be specified up to three
 	times, each resulting in an additional 90 degrees clockwise rotation.

cage.c

@@ -192,6 +192,8 @@ usage(FILE *file, const char *cage)
 		" -h\t Display this help message\n"
 		" -m extend Extend the display across all connected outputs (default)\n"
 		" -m last Use only the last connected output\n"
+		" -e auto Automatically place newly connected outputs (default)\n"
+		" -e right Place newly connected outputs on the right\n"
 		" -r\t Rotate the output 90 degrees clockwise, specify up to three times\n"
 		" -s\t Allow VT switching\n"
 		" -v\t Show the version number and exit\n"
@@ -204,7 +206,7 @@ static bool
 parse_args(struct cg_server *server, int argc, char *argv[])
 {
 	int c;
-	while ((c = getopt(argc, argv, "dhm:rsv")) != -1) {
+	while ((c = getopt(argc, argv, "dhm:e:rsv")) != -1) {
 		switch (c) {
 		case 'd':
 			server->xdg_decoration = true;
@@ -219,6 +221,13 @@ parse_args(struct cg_server *server, int argc, char *argv[])
 				server->output_mode = CAGE_MULTI_OUTPUT_MODE_EXTEND;
 			}
 			break;
+		case 'e':
+			if (strcmp(optarg, "right") == 0) {
+				server->output_extend_mode = CAGE_OUTPUT_EXTEND_MODE_RIGHT;
+			} else if (strcmp(optarg, "auto") == 0) {
+				server->output_extend_mode = CAGE_OUTPUT_EXTEND_MODE_AUTO;
+			}
+			break;
 		case 'r':
 			server->output_transform++;
 			if (server->output_transform > WL_OUTPUT_TRANSFORM_270) {

output.c

@@ -52,7 +52,23 @@ output_enable(struct cg_output *output)
 	 * duplicate the enabled property in cg_output. */
 	wlr_log(WLR_DEBUG, "Enabling output %s", wlr_output->name);
 
+	if (output->server->output_extend_mode == CAGE_OUTPUT_EXTEND_MODE_RIGHT) {
+		int max_x = 0, max_x_y = 0;
+		struct wlr_output_layout_output *l_output;
+		wl_list_for_each (l_output, &output->server->output_layout->outputs, link) {
+			int width, height;
+			wlr_output_effective_resolution(l_output->output, &width, &height);
+			if (l_output->x + width > max_x) {
+				max_x = l_output->x + width;
+				max_x_y = l_output->y;
+			}
+		}
+
+		wlr_output_layout_add(output->server->output_layout, wlr_output, max_x, max_x_y);
+	} else {
 		wlr_output_layout_add_auto(output->server->output_layout, wlr_output);
+	}
+
 	wlr_output_enable(wlr_output, true);
 	wlr_output_commit(wlr_output);
 

server.h

@@ -21,6 +21,11 @@ enum cg_multi_output_mode {
 	CAGE_MULTI_OUTPUT_MODE_LAST,
 };
 
+enum cg_output_extend_mode {
+	CAGE_OUTPUT_EXTEND_MODE_AUTO,
+	CAGE_OUTPUT_EXTEND_MODE_RIGHT,
+};
+
 struct cg_server {
 	struct wl_display *wl_display;
 	struct wl_list views;
@@ -35,6 +40,7 @@ struct cg_server {
 	struct wl_list inhibitors;
 
 	enum cg_multi_output_mode output_mode;
+	enum cg_output_extend_mode output_extend_mode;
 	struct wlr_output_layout *output_layout;
 	struct wlr_scene *scene;
 	/* Includes disabled outputs; depending on the output_mode