diff --git a/internal/restorable/image.go b/internal/restorable/image.go
index 129642181..21cd5058f 100644
--- a/internal/restorable/image.go
+++ b/internal/restorable/image.go
@@ -17,7 +17,6 @@ package restorable
 import (
 	"fmt"
 	"image"
-	"math"
 
 	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
@@ -270,14 +269,7 @@ func (i *Image) BasePixelsForTesting() *Pixels {
 // makeStale makes the image stale.
 func (i *Image) makeStale(rect image.Rectangle) {
 	i.stale = true
-
-	r := i.staleRegion
-	r = r.Union(i.basePixels.Region())
-	for _, d := range i.drawTrianglesHistory {
-		r = r.Union(regionToRectangle(d.dstRegion))
-	}
-	r = r.Union(rect)
-	i.staleRegion = r
+	i.staleRegion = i.staleRegion.Union(i.basePixels.Region()).Union(rect)
 
 	i.basePixels = Pixels{}
 	i.clearDrawTrianglesHistory()
@@ -346,7 +338,7 @@ func (i *Image) WritePixels(pixels []byte, x, y, width, height int) {
 
 	// Records for DrawTriangles cannot come before records for WritePixels.
 	if len(i.drawTrianglesHistory) > 0 {
-		i.makeStale(image.Rect(x, y, x+width, y+height))
+		i.makeStale(image.Rect(0, 0, i.width, i.height))
 		return
 	}
 
@@ -394,9 +386,8 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, offsets [g
 		}
 	}
 
-	// Even if the image is already stale, call makeStale to extend the stale region.
-	if srcstale || !needsRestoring() || !i.needsRestoring() || i.stale {
-		i.makeStale(regionToRectangle(dstRegion))
+	if srcstale || !needsRestoring() || !i.needsRestoring() {
+		i.makeStale(image.Rect(0, 0, i.width, i.height))
 	} else {
 		i.appendDrawTrianglesHistory(srcs, offsets, vertices, indices, blend, dstRegion, srcRegion, shader, uniforms, evenOdd)
 	}
@@ -414,13 +405,13 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, offsets [g
 // appendDrawTrianglesHistory appends a draw-image history item to the image.
 func (i *Image) appendDrawTrianglesHistory(srcs [graphics.ShaderImageCount]*Image, offsets [graphics.ShaderImageCount - 1][2]float32, vertices []float32, indices []uint16, blend graphicsdriver.Blend, dstRegion, srcRegion graphicsdriver.Region, shader *Shader, uniforms []uint32, evenOdd bool) {
 	if i.stale || !i.needsRestoring() {
-		panic("restorable: an image must not be stale or need restoring at appendDrawTrianglesHistory")
+		return
 	}
 
 	// TODO: Would it be possible to merge draw image history items?
 	const maxDrawTrianglesHistoryCount = 1024
 	if len(i.drawTrianglesHistory)+1 > maxDrawTrianglesHistoryCount {
-		i.makeStale(regionToRectangle(dstRegion))
+		i.makeStale(image.Rect(0, 0, i.width, i.height))
 		return
 	}
 	// All images must be resolved and not stale each after frame.
@@ -476,8 +467,7 @@ func (i *Image) makeStaleIfDependingOn(target *Image) {
 		return
 	}
 	if i.dependsOn(target) {
-		// There is no new region to make stale.
-		i.makeStale(image.Rectangle{})
+		i.makeStale(image.Rect(0, 0, i.width, i.height))
 	}
 }
 
@@ -487,21 +477,16 @@ func (i *Image) makeStaleIfDependingOnShader(shader *Shader) {
 		return
 	}
 	if i.dependsOnShader(shader) {
-		// There is no new region to make stale.
-		i.makeStale(image.Rectangle{})
+		i.makeStale(image.Rect(0, 0, i.width, i.height))
 	}
 }
 
 // readPixelsFromGPU reads the pixels from GPU and resolves the image's 'stale' state.
 func (i *Image) readPixelsFromGPU(graphicsDriver graphicsdriver.Graphics) error {
-	var r image.Rectangle
-	if i.stale {
-		i.basePixels = Pixels{}
-		r = i.staleRegion
-	} else {
-		for _, d := range i.drawTrianglesHistory {
-			r = r.Union(regionToRectangle(d.dstRegion))
-		}
+	i.basePixels = Pixels{}
+	r := i.staleRegion
+	if len(i.drawTrianglesHistory) > 0 {
+		r = image.Rect(0, 0, i.width, i.height)
 	}
 	if !r.Empty() {
 		var pix []byte
@@ -693,11 +678,3 @@ func (i *Image) clearDrawTrianglesHistory() {
 func (i *Image) InternalSize() (int, int) {
 	return i.image.InternalSize()
 }
-
-func regionToRectangle(region graphicsdriver.Region) image.Rectangle {
-	return image.Rect(
-		int(math.Floor(float64(region.X))),
-		int(math.Floor(float64(region.Y))),
-		int(math.Ceil(float64(region.X+region.Width))),
-		int(math.Ceil(float64(region.Y+region.Height))))
-}
diff --git a/internal/restorable/images_test.go b/internal/restorable/images_test.go
index 55302ff09..2d742aad6 100644
--- a/internal/restorable/images_test.go
+++ b/internal/restorable/images_test.go
@@ -1129,36 +1129,3 @@ func TestDrawTrianglesAndReadPixels(t *testing.T) {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
 }
-
-func TestWritePixelsAndDrawTriangles(t *testing.T) {
-	src := restorable.NewImage(1, 1, restorable.ImageTypeRegular)
-	dst := restorable.NewImage(2, 1, restorable.ImageTypeRegular)
-
-	src.WritePixels([]byte{0x80, 0x80, 0x80, 0x80}, 0, 0, 1, 1)
-
-	// Call WritePixels first.
-	dst.WritePixels([]byte{0x40, 0x40, 0x40, 0x40}, 0, 0, 1, 1)
-
-	// Call DrawTriangles at a different region second.
-	vs := quadVertices(src, 1, 1, 1, 0)
-	is := graphics.QuadIndices()
-	dr := graphicsdriver.Region{
-		X:      1,
-		Y:      0,
-		Width:  1,
-		Height: 1,
-	}
-	dst.DrawTriangles([graphics.ShaderImageCount]*restorable.Image{src}, [graphics.ShaderImageCount - 1][2]float32{}, vs, is, graphicsdriver.BlendSourceOver, dr, graphicsdriver.Region{}, restorable.NearestFilterShader, nil, false)
-
-	// Get the pixels.
-	pix := make([]byte, 4*2*1)
-	if err := dst.ReadPixels(ui.GraphicsDriverForTesting(), pix, 0, 0, 2, 1); err != nil {
-		t.Fatal(err)
-	}
-	if got, want := (color.RGBA{R: pix[0], G: pix[1], B: pix[2], A: pix[3]}), (color.RGBA{R: 0x40, G: 0x40, B: 0x40, A: 0x40}); !sameColors(got, want, 1) {
-		t.Errorf("got: %v, want: %v", got, want)
-	}
-	if got, want := (color.RGBA{R: pix[4], G: pix[5], B: pix[6], A: pix[7]}), (color.RGBA{R: 0x80, G: 0x80, B: 0x80, A: 0x80}); !sameColors(got, want, 1) {
-		t.Errorf("got: %v, want: %v", got, want)
-	}
-}