graphics: Try to restore the context on browsers (WIP)

internal/graphics/image.go

@@ -180,7 +180,7 @@ func (i *Image) ReplacePixels(p []uint8) error {
 }
 
 func (i *Image) IsInvalidated(context *opengl.Context) bool {
-	return context.IsContextLost(i.texture.native)
+	return !context.IsTexture(i.texture.native)
 }
 
 func (i *Image) createFramebufferIfNeeded(context *opengl.Context) (*framebuffer, error) {

internal/opengl/context_desktop.go

@@ -514,7 +514,3 @@ func (c *Context) Flush() {
 		return nil
 	})
 }
-
-func (c *Context) IsContextLost(t Texture) bool {
-	return !c.IsTexture(t)
-}

internal/opengl/context_js.go

@@ -87,6 +87,7 @@ func init() {
 
 type context struct {
 	gl            *webgl.Context
+	loseContext   *js.Object
 	lastProgramID programID
 }
 
@@ -116,6 +117,15 @@ func NewContext() (*Context, error) {
 	}
 	c := &Context{}
 	c.gl = gl
+	// Getting an extension might fail after the context is lost, so
+	// it is required to get the extension here.
+	c.loseContext = gl.GetExtension("WEBGL_lose_context")
+	if c.loseContext != nil {
+		// This testing function name is temporary.
+		js.Global.Set("_ebiten_loseContextForTesting", func() {
+			c.loseContext.Call("loseContext")
+		})
+	}
 	return c, nil
 }
 
@@ -408,7 +418,13 @@ func (c *Context) Flush() {
 	gl.Flush()
 }
 
-func (c *Context) IsContextLost(t Texture) bool {
+func (c *Context) IsContextLost() bool {
 	gl := c.gl
 	return gl.IsContextLost()
 }
+
+func (c *Context) RestoreContext() {
+	if c.loseContext != nil {
+		c.loseContext.Call("restoreContext")
+	}
+}

internal/opengl/context_mobile.go

@@ -430,7 +430,3 @@ func (c *Context) Flush() {
 	gl := c.gl
 	gl.Flush()
 }
-
-func (c *Context) IsContextLost(t Texture) bool {
-	return !c.IsTexture(t)
-}

internal/ui/ui_js.go

@@ -27,17 +27,15 @@ import (
 var canvas *js.Object
 
 type userInterface struct {
-	scale           float64
-	deviceScale     float64
-	sizeChanged     bool
-	contextRestored bool
-	windowFocus     bool
+	scale       float64
+	deviceScale float64
+	sizeChanged bool
+	windowFocus bool
 }
 
 var currentUI = &userInterface{
-	sizeChanged:     true,
-	contextRestored: true,
-	windowFocus:     true,
+	sizeChanged: true,
+	windowFocus: true,
 }
 
 // NOTE: This returns true even when the browser is not active.
@@ -74,8 +72,8 @@ func (u *userInterface) update(g GraphicsContext) error {
 	if !u.windowFocus {
 		return nil
 	}
-	if !u.contextRestored {
-		return nil
+	if glContext.IsContextLost() {
+		glContext.RestoreContext()
 	}
 	currentInput.updateGamepads()
 	if u.sizeChanged {
@@ -235,12 +233,11 @@ func initialize() error {
 
 	canvas.Call("addEventListener", "webglcontextlost", func(e *js.Object) {
 		e.Call("preventDefault")
-		currentUI.contextRestored = false
 	})
 	canvas.Call("addEventListener", "webglcontextrestored", func(e *js.Object) {
-		// TODO: Call preventDefault?
-		currentUI.contextRestored = true
+		// Do nothing.
 	})
+
 	return nil
 }