diff --git a/internal/ui/init.go b/internal/ui/init.go
deleted file mode 100644
index 2876c7c36..000000000
--- a/internal/ui/init.go
+++ /dev/null
@@ -1,33 +0,0 @@
-// Copyright 2016 Hajime Hoshi
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package ui
-
-import (
-	"github.com/hajimehoshi/ebiten/internal/graphics/opengl"
-)
-
-var glContext *opengl.Context
-
-func GLContext() *opengl.Context {
-	return glContext
-}
-
-func init() {
-	var err error
-	glContext, err = initialize()
-	if err != nil {
-		panic(err)
-	}
-}
diff --git a/internal/ui/ui_glfw.go b/internal/ui/ui_glfw.go
index cd87b5ade..ee71fccc6 100644
--- a/internal/ui/ui_glfw.go
+++ b/internal/ui/ui_glfw.go
@@ -44,11 +44,21 @@ func CurrentUI() UserInterface {
 	return currentUI
 }
 
-func initialize() (*opengl.Context, error) {
+func GLContext() *opengl.Context {
+	return currentUI.context
+}
+
+func init() {
+	if err := initialize(); err != nil {
+		panic(err)
+	}
+}
+
+func initialize() error {
 	runtime.LockOSThread()
 
 	if err := glfw.Init(); err != nil {
-		return nil, err
+		return err
 	}
 	glfw.WindowHint(glfw.Visible, glfw.False)
 	glfw.WindowHint(glfw.Resizable, glfw.False)
@@ -58,7 +68,7 @@ func initialize() (*opengl.Context, error) {
 	// As start, create an window with temporary size to create OpenGL context thread.
 	window, err := glfw.CreateWindow(16, 16, "", nil, nil)
 	if err != nil {
-		return nil, err
+		return err
 	}
 
 	u := &userInterface{
@@ -81,10 +91,9 @@ func initialize() (*opengl.Context, error) {
 	}()
 	currentUI = u
 	if err := <-ch; err != nil {
-		return nil, err
+		return err
 	}
-
-	return u.context, nil
+	return nil
 }
 
 func Main() error {
diff --git a/internal/ui/ui_js.go b/internal/ui/ui_js.go
index a86c39505..811e60021 100644
--- a/internal/ui/ui_js.go
+++ b/internal/ui/ui_js.go
@@ -136,14 +136,30 @@ func touchEventToTouches(e *js.Object) []touch {
 	return t
 }
 
-func initialize() (*opengl.Context, error) {
+var glContext *opengl.Context
+
+func GLContext() *opengl.Context {
+	if glContext != nil {
+		return glContext
+	}
+	var err error
+	glContext, err = opengl.NewContext()
+	if err != nil {
+		panic(err)
+	}
+	return glContext
+}
+
+func init() {
+	if err := initialize(); err != nil {
+		panic(err)
+	}
+}
+
+func initialize() error {
 	// Do nothing in node.js.
 	if js.Global.Get("require") != js.Undefined {
-		c, err := opengl.NewContext()
-		if err != nil {
-			return nil, err
-		}
-		return c, nil
+		return nil
 	}
 
 	doc := js.Global.Get("document")
@@ -255,12 +271,7 @@ func initialize() (*opengl.Context, error) {
 		close(currentUI.contextRestored)
 		currentUI.contextRestored = nil
 	})
-
-	c, err := opengl.NewContext()
-	if err != nil {
-		return nil, err
-	}
-	return c, nil
+	return nil
 }
 
 func setMouseCursorFromEvent(e *js.Object) {
diff --git a/internal/ui/ui_mobile.go b/internal/ui/ui_mobile.go
index e9fdfdc7f..47b63a5c2 100644
--- a/internal/ui/ui_mobile.go
+++ b/internal/ui/ui_mobile.go
@@ -24,8 +24,18 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/graphics/opengl"
 )
 
-func initialize() (*opengl.Context, error) {
-	return opengl.NewContext()
+var glContext *opengl.Context
+
+func GLContext() *opengl.Context {
+	if glContext != nil {
+		return glContext
+	}
+	var err error
+	glContext, err = opengl.NewContext()
+	if err != nil {
+		panic(err)
+	}
+	return glContext
 }
 
 func Main() error {
diff --git a/pixels.go b/pixels.go
index feed15c4a..ce1ac7f4b 100644
--- a/pixels.go
+++ b/pixels.go
@@ -91,6 +91,7 @@ func (p *pixels) hasHistoryWith(target *Image) bool {
 }
 
 func (p *pixels) resetHistoryIfNeeded(image *graphics.Image, target *Image, context *opengl.Context) error {
+	// TODO: Return error when the main loop is not running yet.
 	if p.drawImageHistory == nil {
 		return nil
 	}