ebiten: Bug fix: Set the state of being volatile on SetClearingScreenSkipped

Fixes #1309

examples/contextlost/main.go

@@ -93,6 +93,10 @@ func (g *Game) Update(screen *ebiten.Image) error {
 		return nil
 	}
 
+	if inpututil.IsKeyJustPressed(ebiten.KeyS) {
+		ebiten.SetClearingScreenSkipped(!ebiten.IsClearingScreenSkipped())
+	}
+
 	g.count++
 	return nil
 }
@@ -107,7 +111,12 @@ func (g *Game) Draw(screen *ebiten.Image) {
 	op.GeoM.Translate(screenWidth/2, screenHeight/2)
 	screen.DrawImage(gophersImage, op)
 
-	ebitenutil.DebugPrint(screen, "Press Space to force to lose/restore the GL context!\n(Browser only)")
+	msg := `Press Space to force to lose/restore the GL context!
+(Browser only)
+
+Press S to switch clearing the screen
+at the beginning of each frame.`
+	ebitenutil.DebugPrint(screen, msg)
 }
 
 func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {

run.go

@@ -112,6 +112,7 @@ func SetClearingScreenSkipped(skipped bool) {
 		v = 1
 	}
 	atomic.StoreInt32(&isClearingScreenSkipped, v)
+	theUIContext.setClearingScreenSkipped(skipped)
 }
 
 // IsClearingScreenSkipped returns true if the frame isn't cleared at the beginning.

uicontext.go

@@ -176,7 +176,7 @@ func (c *uiContext) updateOffscreen() {
 	}
 	if c.offscreen == nil {
 		c.offscreen = newImage(sw, sh, FilterDefault)
-		c.offscreen.mipmap.SetVolatile(true)
+		c.offscreen.mipmap.SetVolatile(!IsClearingScreenSkipped())
 	}
 
 	// The window size is automatically adjusted when Run is used.
@@ -193,6 +193,15 @@ func (c *uiContext) updateOffscreen() {
 	// scale. This is fine since ebiten.ScreenScale will be deprecated.
 }
 
+func (c *uiContext) setClearingScreenSkipped(skipped bool) {
+	c.m.Lock()
+	defer c.m.Unlock()
+
+	if c.offscreen != nil {
+		c.offscreen.mipmap.SetVolatile(!skipped)
+	}
+}
+
 func (c *uiContext) setWindowResizable(resizable bool) {
 	c.m.Lock()
 	defer c.m.Unlock()