diff --git a/internal/ui/run_notsinglethread.go b/internal/ui/run_notsinglethread.go
index 93beb2a1d..e40711c10 100644
--- a/internal/ui/run_notsinglethread.go
+++ b/internal/ui/run_notsinglethread.go
@@ -60,7 +60,7 @@ func (u *UserInterface) Run(game Game) error {
 // u.t is updated to the new thread until runOnAnotherThreadFromMainThread is called.
 //
 // Inside f, another functions that must be called from the main thread can be called safely.
-func (u *UserInterface) runOnAnotherThreadFromMainThread(f func() error) error {
+func (u *UserInterface) runOnAnotherThreadFromMainThread(f func()) {
 	// As this function is called from the main thread, u.t should never be accessed and can be updated here.
 	t := u.t
 	defer func() {
@@ -71,11 +71,9 @@ func (u *UserInterface) runOnAnotherThreadFromMainThread(f func() error) error {
 	u.t = thread.NewOSThread()
 	graphicscommand.SetRenderingThread(u.t)
 
-	var err error
 	go func() {
 		defer u.t.Stop()
-		err = f()
+		f()
 	}()
 	u.t.Loop()
-	return err
 }
diff --git a/internal/ui/run_singlethread.go b/internal/ui/run_singlethread.go
index eb738fea8..2628b86dc 100644
--- a/internal/ui/run_singlethread.go
+++ b/internal/ui/run_singlethread.go
@@ -43,6 +43,6 @@ func (u *UserInterface) Run(game Game) error {
 	return nil
 }
 
-func (u *UserInterface) runOnAnotherThreadFromMainThread(f func() error) error {
-	return f()
+func (u *UserInterface) runOnAnotherThreadFromMainThread(f func()) {
+	f()
 }
diff --git a/internal/ui/ui_glfw.go b/internal/ui/ui_glfw.go
index 54b585dde..57a76ffbf 100644
--- a/internal/ui/ui_glfw.go
+++ b/internal/ui/ui_glfw.go
@@ -109,6 +109,7 @@ type UserInterface struct {
 	defaultFramebufferSizeCallback glfw.FramebufferSizeCallback
 	framebufferSizeCallbackCh      chan struct{}
 
+	// t is the main thread == the rendering thread.
 	t thread.Thread
 	m sync.RWMutex
 }
@@ -701,33 +702,29 @@ func (u *UserInterface) registerWindowSetSizeCallback() {
 				return
 			}
 
-			if err := u.runOnAnotherThreadFromMainThread(func() error {
-				var outsideWidth, outsideHeight float64
-				var deviceScaleFactor float64
+			if width != 0 || height != 0 {
+				w := int(u.dipFromGLFWPixel(float64(width), u.currentMonitor()))
+				h := int(u.dipFromGLFWPixel(float64(height), u.currentMonitor()))
+				u.setWindowSizeInDIP(w, h, u.isFullscreen())
+			}
 
-				u.t.Call(func() {
-					if width != 0 || height != 0 {
-						w := int(u.dipFromGLFWPixel(float64(width), u.currentMonitor()))
-						h := int(u.dipFromGLFWPixel(float64(height), u.currentMonitor()))
-						u.setWindowSizeInDIP(w, h, u.isFullscreen())
-					}
+			outsideWidth, outsideHeight := u.updateSize()
+			deviceScaleFactor := u.deviceScaleFactor(u.currentMonitor())
 
-					outsideWidth, outsideHeight = u.updateSize()
-					deviceScaleFactor = u.deviceScaleFactor(u.currentMonitor())
-				})
+			// In the game's update, u.t.Call might be called.
+			// In order to call it safely, use runOnAnotherThreadFromMainThread.
+			var err error
+			u.runOnAnotherThreadFromMainThread(func() {
 				u.context.layout(outsideWidth, outsideHeight)
-				if err := u.context.forceUpdateFrame(deviceScaleFactor); err != nil {
-					return err
-				}
-				if graphics().IsGL() {
-					u.t.Call(func() {
-						u.swapBuffers()
-					})
-				}
-				return nil
-			}); err != nil {
+				err = u.context.forceUpdateFrame(deviceScaleFactor)
+			})
+			if err != nil {
 				u.err = err
 			}
+
+			if graphics().IsGL() {
+				u.swapBuffers()
+			}
 		})
 	}
 	u.window.SetSizeCallback(u.sizeCallback)