internal/atlas: bug fix: the offscreen must be an independent image

Closes #1938

internal/atlas/image.go

@@ -218,11 +218,12 @@ func init() {
 
 // Image is a renctangle pixel set that might be on an atlas.
 type Image struct {
-	width    int
-	height   int
-	disposed bool
-	volatile bool
-	screen   bool
+	width       int
+	height      int
+	disposed    bool
+	independent bool
+	volatile    bool
+	screen      bool
 
 	backend *backend
 
@@ -694,6 +695,10 @@ func NewImage(width, height int) *Image {
 	}
 }
 
+func (i *Image) SetIndependent(independent bool) {
+	i.independent = independent
+}
+
 func (i *Image) SetVolatile(volatile bool) {
 	i.volatile = volatile
 	if i.backend == nil {
@@ -709,6 +714,9 @@ func (i *Image) canBePutOnAtlas() bool {
 	if minSize == 0 || maxSize == 0 {
 		panic("atlas: minSize or maxSize must be initialized")
 	}
+	if i.independent {
+		return false
+	}
 	if i.volatile {
 		return false
 	}

internal/buffered/image.go

@@ -65,6 +65,18 @@ func (i *Image) initialize(width, height int) {
 	i.height = height
 }
 
+func (i *Image) SetIndependent(volatile bool) {
+	if maybeCanAddDelayedCommand() {
+		if tryAddDelayedCommand(func() error {
+			i.SetIndependent(volatile)
+			return nil
+		}) {
+			return
+		}
+	}
+	i.img.SetIndependent(volatile)
+}
+
 func (i *Image) SetVolatile(volatile bool) {
 	if maybeCanAddDelayedCommand() {
 		if tryAddDelayedCommand(func() error {

internal/mipmap/mipmap.go

@@ -59,6 +59,10 @@ func NewScreenFramebufferMipmap(width, height int) *Mipmap {
 	}
 }
 
+func (m *Mipmap) SetIndependent(volatile bool) {
+	m.orig.SetIndependent(volatile)
+}
+
 func (m *Mipmap) SetVolatile(volatile bool) {
 	m.volatile = volatile
 	if m.volatile {

uicontext.go

@@ -93,6 +93,12 @@ func (c *uiContext) updateOffscreen() {
 	if c.offscreen == nil {
 		c.offscreen = NewImage(ow, oh)
 		c.offscreen.mipmap.SetVolatile(IsScreenClearedEveryFrame())
+
+		// Keep the offscreen an independent image from an atlas (#1938).
+		// The shader program for the screen is special and doesn't work well with an image on an atlas.
+		// An image on an atlas is surrounded by a transparent edge,
+		// and the shader program unexpectedly picks the pixel on the edges.
+		c.offscreen.mipmap.SetIndependent(true)
 	}
 }