Implement hide_edge_borders smart (like in i3 4.13)

include/sway/config.h

@@ -178,7 +178,8 @@ enum edge_border_types {
 	E_NONE,         /**< Don't hide edge borders */
 	E_VERTICAL,     /**< hide vertical edge borders */
 	E_HORIZONTAL,   /**< hide horizontal edge borders */
-	E_BOTH		/**< hide vertical and horizontal edge borders */
+	E_BOTH,		/**< hide vertical and horizontal edge borders */
+	E_SMART		/**< hide both if precisely one window is present in workspace */
 };
 
 enum command_context {

sway/commands/hide_edge_borders.c

@@ -15,6 +15,8 @@ struct cmd_results *cmd_hide_edge_borders(int argc, char **argv) {
 		config->hide_edge_borders = E_HORIZONTAL;
 	} else if (strcasecmp(argv[0], "both") == 0) {
 		config->hide_edge_borders = E_BOTH;
+	} else if (strcasecmp(argv[0], "smart") == 0) {
+		config->hide_edge_borders = E_SMART;
 	} else {
 		return cmd_results_new(CMD_INVALID, "hide_edge_borders",
 				"Expected 'hide_edge_borders <none|vertical|horizontal|both>'");

sway/layout.c

@@ -665,6 +665,13 @@ void update_geometry(swayc_t *container) {
 					border_bottom = 0;
 				}
 			}
+
+			if (config->hide_edge_borders == E_SMART && workspace->children->length == 1) {
+				border_top = 0;
+				border_bottom = 0;
+				border_left = 0;
+				border_right = 0;
+			}
 		}
 
 		int title_bar_height = config->font_height + 4; //borders + padding