Fallback to other output modes if preferred mode fails

Sometimes the preferred mode is not available due to hardware
constraints (e.g. GPU or cable bandwidth limitations). In these
cases it's better to fallback to lower modes than to end up with
a black screen.

sway/config/output.c

@@ -357,8 +357,26 @@ static void queue_output_config(struct output_config *oc,
 		set_mode(wlr_output, oc->width, oc->height,
 			oc->refresh_rate, oc->custom_mode == 1);
 	} else if (!wl_list_empty(&wlr_output->modes)) {
-		struct wlr_output_mode *mode = wlr_output_preferred_mode(wlr_output);
-		wlr_output_set_mode(wlr_output, mode);
+		sway_log(SWAY_DEBUG, "Set preferred mode");
+		struct wlr_output_mode *preferred_mode =
+			wlr_output_preferred_mode(wlr_output);
+		wlr_output_set_mode(wlr_output, preferred_mode);
+
+		if (!wlr_output_test(wlr_output)) {
+			sway_log(SWAY_DEBUG, "Preferred mode rejected, "
+				"falling back to another mode");
+			struct wlr_output_mode *mode;
+			wl_list_for_each(mode, &wlr_output->modes, link) {
+				if (mode == preferred_mode) {
+					continue;
+				}
+
+				wlr_output_set_mode(wlr_output, mode);
+				if (wlr_output_test(wlr_output)) {
+					break;
+				}
+			}
+		}
 	}
 
 	if (oc && (oc->subpixel != WL_OUTPUT_SUBPIXEL_UNKNOWN || config->reloading)) {