diff --git a/internal/ui/ui_glfw.go b/internal/ui/ui_glfw.go
index 833ac7a4a..12f778d2f 100644
--- a/internal/ui/ui_glfw.go
+++ b/internal/ui/ui_glfw.go
@@ -320,10 +320,13 @@ func IsRunnableInBackground() bool {
 func SetVsyncEnabled(enabled bool) {
 	u := currentUI
 	if !u.isRunning() {
-		_ = u.runOnMainThread(func() error {
-			u.vsync = enabled
-			return nil
-		})
+		// In general, m is used for locking init* values.
+		// m is not used for updating vsync in setScreenSize so far, but
+		// it should be OK since any goroutines can't reach here when
+		// the game already starts and setScreenSize can be called.
+		u.m.Lock()
+		u.vsync = enabled
+		u.m.Unlock()
 		return
 	}
 	_ = u.runOnMainThread(func() error {