cmd/syncthing: Delay browser start until the GUI is ready (fixes #3619)

cmd/syncthing/main.go

@@ -947,6 +947,7 @@ func setupGUI(mainService *suture.Supervisor, cfg *config.Wrapper, m *model.Mode
 	if cfg.Options().StartBrowser && !runtimeOptions.noBrowser && !runtimeOptions.stRestarting {
 		// Can potentially block if the utility we are invoking doesn't
 		// fork, and just execs, hence keep it in it's own routine.
+		<-api.startedOnce
 		go openURL(guiCfg.URL())
 	}
 }