// SPDX-License-Identifier: Unlicense OR MIT

package app

import (
	"fmt"
	"image"
	"image/color"
	"io"
	"strings"
	"syscall/js"
	"time"
	"unicode"
	"unicode/utf8"

	"gioui.org/internal/f32color"
	"gioui.org/op"

	"gioui.org/f32"
	"gioui.org/io/event"
	"gioui.org/io/key"
	"gioui.org/io/pointer"
	"gioui.org/io/system"
	"gioui.org/io/transfer"
	"gioui.org/unit"
)

type JSViewEvent struct {
	Element js.Value
}

type contextStatus int

const (
	contextStatusOkay contextStatus = iota
	contextStatusLost
	contextStatusRestored
)

type window struct {
	window                js.Value
	document              js.Value
	head                  js.Value
	clipboard             js.Value
	cnv                   js.Value
	tarea                 js.Value
	w                     *callbacks
	redraw                js.Func
	clipboardCallback     js.Func
	requestAnimationFrame js.Value
	browserHistory        js.Value
	visualViewport        js.Value
	screenOrientation     js.Value
	cleanfuncs            []func()
	touches               []js.Value
	composing             int
	lastCursor            int
	requestFocus          bool

	config    Config
	inset     f32.Point
	scale     float32
	animating bool
	// animRequested tracks whether a requestAnimationFrame callback
	// is pending.
	animRequested bool
	wakeups       chan struct{}

	contextStatus contextStatus
}

func newWindow(win *callbacks, options []Option) {
	doc := js.Global().Get("document")
	cont := getContainer(doc)
	cnv := createCanvas(doc)
	cont.Call("appendChild", cnv)
	tarea := createTextArea(doc)
	cont.Call("appendChild", tarea)
	w := &window{
		cnv:       cnv,
		document:  doc,
		tarea:     tarea,
		window:    js.Global().Get("window"),
		head:      doc.Get("head"),
		clipboard: js.Global().Get("navigator").Get("clipboard"),
		wakeups:   make(chan struct{}, 1),
		w:         win,
		composing: -1,
	}
	w.w.SetDriver(w)
	w.requestAnimationFrame = w.window.Get("requestAnimationFrame")
	w.browserHistory = w.window.Get("history")
	w.visualViewport = w.window.Get("visualViewport")
	if w.visualViewport.IsUndefined() {
		w.visualViewport = w.window
	}
	if screen := w.window.Get("screen"); screen.Truthy() {
		w.screenOrientation = screen.Get("orientation")
	}
	w.redraw = w.funcOf(func(this js.Value, args []js.Value) interface{} {
		w.draw(false)
		return nil
	})
	w.clipboardCallback = w.funcOf(func(this js.Value, args []js.Value) interface{} {
		content := args[0].String()
		w.processEvent(transfer.DataEvent{
			Type: "application/text",
			Open: func() io.ReadCloser {
				return io.NopCloser(strings.NewReader(content))
			},
		})
		return nil
	})
	w.addEventListeners()
	w.addHistory()

	w.Configure(options)
	w.blur()
	w.processEvent(JSViewEvent{Element: cont})
	w.resize()
	w.draw(true)
}

func getContainer(doc js.Value) js.Value {
	cont := doc.Call("getElementById", "giowindow")
	if !cont.IsNull() {
		return cont
	}
	cont = doc.Call("createElement", "DIV")
	doc.Get("body").Call("appendChild", cont)
	return cont
}

func createTextArea(doc js.Value) js.Value {
	tarea := doc.Call("createElement", "textarea")
	style := tarea.Get("style")
	// Position absolute so left/top coordinates actually place the element
	style.Set("position", "absolute")
	style.Set("width", "1px")
	style.Set("height", "1px")
	style.Set("opacity", "0")
	style.Set("border", "0")
	style.Set("padding", "0")
	tarea.Set("autocomplete", "off")
	tarea.Set("autocorrect", "off")
	tarea.Set("autocapitalize", "off")
	tarea.Set("spellcheck", false)
	// Enable multiline text input for better composition support on some browsers.
	tarea.Set("rows", 1)
	style.Set("resize", "none")
	style.Set("overflow", "hidden")
	style.Set("white-space", "pre-wrap")
	style.Set("word-break", "normal")
	return tarea
}

func createCanvas(doc js.Value) js.Value {
	cnv := doc.Call("createElement", "canvas")
	style := cnv.Get("style")
	style.Set("position", "fixed")
	style.Set("width", "100%")
	style.Set("height", "100%")
	return cnv
}

func (w *window) cleanup() {
	// Cleanup in the opposite order of
	// construction.
	for i := len(w.cleanfuncs) - 1; i >= 0; i-- {
		w.cleanfuncs[i]()
	}
	w.cleanfuncs = nil
}

func (w *window) addEventListeners() {
	w.addEventListener(w.cnv, "webglcontextlost", func(this js.Value, args []js.Value) interface{} {
		args[0].Call("preventDefault")
		w.contextStatus = contextStatusLost
		return nil
	})
	w.addEventListener(w.cnv, "webglcontextrestored", func(this js.Value, args []js.Value) interface{} {
		args[0].Call("preventDefault")
		w.contextStatus = contextStatusRestored

		// Resize is required to force update the canvas content when restored.
		w.cnv.Set("width", 0)
		w.cnv.Set("height", 0)
		w.resize()
		w.draw(true)
		return nil
	})
	w.addEventListener(w.visualViewport, "resize", func(this js.Value, args []js.Value) interface{} {
		w.resize()
		w.draw(true)
		return nil
	})
	w.addEventListener(w.window, "contextmenu", func(this js.Value, args []js.Value) interface{} {
		args[0].Call("preventDefault")
		return nil
	})
	w.addEventListener(w.window, "popstate", func(this js.Value, args []js.Value) interface{} {
		if w.processEvent(key.Event{Name: key.NameBack}) {
			return w.browserHistory.Call("forward")
		}
		return w.browserHistory.Call("back")
	})
	w.addEventListener(w.cnv, "mousemove", func(this js.Value, args []js.Value) interface{} {
		w.pointerEvent(pointer.Move, 0, 0, args[0])
		return nil
	})
	w.addEventListener(w.cnv, "mousedown", func(this js.Value, args []js.Value) interface{} {
		w.pointerEvent(pointer.Press, 0, 0, args[0])
		if w.requestFocus {
			w.focus()
			w.requestFocus = false
		}
		return nil
	})
	w.addEventListener(w.cnv, "mouseup", func(this js.Value, args []js.Value) interface{} {
		w.pointerEvent(pointer.Release, 0, 0, args[0])
		return nil
	})
	w.addEventListener(w.cnv, "wheel", func(this js.Value, args []js.Value) interface{} {
		e := args[0]
		dx, dy := e.Get("deltaX").Float(), e.Get("deltaY").Float()
		// horizontal scroll if shift is pressed.
		if e.Get("shiftKey").Bool() {
			dx, dy = dy, dx
		}
		mode := e.Get("deltaMode").Int()
		switch mode {
		case 0x01: // DOM_DELTA_LINE
			dx *= 10
			dy *= 10
		case 0x02: // DOM_DELTA_PAGE
			dx *= 120
			dy *= 120
		}
		w.pointerEvent(pointer.Scroll, float32(dx), float32(dy), e)
		return nil
	})
	w.addEventListener(w.cnv, "touchstart", func(this js.Value, args []js.Value) interface{} {
		w.touchEvent(pointer.Press, args[0])
		if w.requestFocus {
			w.focus() // iOS can only focus inside a Touch event.
			w.requestFocus = false
		}
		return nil
	})
	w.addEventListener(w.cnv, "touchend", func(this js.Value, args []js.Value) interface{} {
		w.touchEvent(pointer.Release, args[0])
		return nil
	})
	w.addEventListener(w.cnv, "touchmove", func(this js.Value, args []js.Value) interface{} {
		w.touchEvent(pointer.Move, args[0])
		return nil
	})
	w.addEventListener(w.cnv, "touchcancel", func(this js.Value, args []js.Value) interface{} {
		// Cancel all touches even if only one touch was cancelled.
		for i := range w.touches {
			w.touches[i] = js.Null()
		}
		w.touches = w.touches[:0]
		w.processEvent(pointer.Event{
			Kind:   pointer.Cancel,
			Source: pointer.Touch,
		})
		return nil
	})
	w.addEventListener(w.tarea, "focus", func(this js.Value, args []js.Value) interface{} {
		w.config.Focused = true
		w.processEvent(ConfigEvent{Config: w.config})
		return nil
	})
	w.addEventListener(w.tarea, "blur", func(this js.Value, args []js.Value) interface{} {
		if w.composing != -1 {
			// If we're composing, try to cancel.
			// On Javascript is not possible to cancel the composition once started.
			w.w.SetComposingRegion(key.Range{Start: -1, End: -1})
			w.composing = -1
		}

		w.config.Focused = false
		w.lastCursor = 0 // Reset cursor tracking on blur
		w.processEvent(ConfigEvent{Config: w.config})
		w.blur()
		return nil
	})
	w.addEventListener(w.tarea, "keydown", func(this js.Value, args []js.Value) interface{} {
		w.keyEvent(args[0], key.Press)
		return nil
	})
	w.addEventListener(w.tarea, "keyup", func(this js.Value, args []js.Value) interface{} {
		w.keyEvent(args[0], key.Release)
		return nil
	})
	w.addEventListener(w.tarea, "compositionstart", func(this js.Value, args []js.Value) interface{} {
		st := w.w.EditorState()
		sel := st.Selection.Range
		if sel.Start == -1 {
			sel.Start = 0
			sel.End = 0
		}
		w.w.SetEditorSnippet(key.Range{Start: sel.Start, End: sel.End})
		w.composing = sel.Start
		return nil
	})
	w.addEventListener(w.tarea, "compositionend", func(this js.Value, args []js.Value) interface{} {
		finalText := w.tarea.Get("value").String()

		if w.composing != -1 && finalText != "" {
			// Replace the entire composition range with the final text.
			compEnd := w.composing + utf8.RuneCountInString(finalText)
			replaceRange := key.Range{Start: w.composing, End: compEnd}
			w.w.EditorReplace(replaceRange, finalText)
			w.w.SetComposingRegion(key.Range{Start: -1, End: -1})

			// Position cursor after the final composition text.
			newEnd := w.composing + utf8.RuneCountInString(finalText)
			w.w.SetEditorSelection(key.Range{Start: newEnd, End: newEnd})
		}

		w.composing = -1
		w.tarea.Set("value", "")
		return nil
	})
	w.addEventListener(w.tarea, "input", func(this js.Value, args []js.Value) interface{} {
		e := args[0]
		inputType := e.Get("inputType").String()

		dataVal := e.Get("data")
		var data string
		if dataVal.Truthy() {
			data = dataVal.String()
		}

		// Get the current textarea value.
		tareaValue := w.tarea.Get("value").String()
		st := w.w.EditorState()

		sel := st.Selection.Range
		var absStart, absEnd int

		snippetStart := st.Snippet.Range.Start
		snippetEnd := st.Snippet.Range.End

		cursorPos := sel.Start
		selectionEnd := sel.End
		if cursorPos < 0 {
			cursorPos = 0
			selectionEnd = 0
		}

		// Check if we need to expand the snippet to include the range.
		if st.Snippet.Range.Start == 0 && st.Snippet.Range.End == 0 && tareaValue != "" {
			// Empty snippet - set it to include the selection/cursor.
			w.w.SetEditorSnippet(key.Range{Start: cursorPos, End: selectionEnd})
			absStart = cursorPos
			absEnd = selectionEnd
		} else if cursorPos < snippetStart || selectionEnd > snippetEnd {
			// Selection is outside the snippet
			newStart := snippetStart
			newEnd := snippetEnd
			if cursorPos < newStart {
				newStart = cursorPos
			}
			if selectionEnd > newEnd {
				newEnd = selectionEnd
			}
			w.w.SetEditorSnippet(key.Range{Start: newStart, End: newEnd})
			// Refresh state after snippet update.
			st = w.w.EditorState()
			// Use the selection range directly.
			absStart = cursorPos
			absEnd = selectionEnd
		} else {
			// Selection is within snippet to absolute positions.
			absStart = cursorPos
			absEnd = selectionEnd
		}

		switch inputType {
		case "insertCompositionText":
			if w.composing == -1 {
				break
			}

			compEnd := absEnd
			if compEnd < w.composing {
				compEnd = w.composing
			}
			replaceRange := key.Range{Start: w.composing, End: compEnd}
			w.w.EditorReplace(replaceRange, data)

			newEnd := w.composing + utf8.RuneCountInString(data)
			w.w.SetComposingRegion(key.Range{Start: w.composing, End: newEnd})
			w.w.SetEditorSelection(key.Range{Start: newEnd, End: newEnd})

		case "deleteContentBackward", "deleteContentForward", "deleteByCut":
			if w.composing != -1 {
				compEnd := w.composing + utf8.RuneCountInString(tareaValue)
				replaceRange := key.Range{Start: w.composing, End: compEnd}
				w.w.EditorReplace(replaceRange, tareaValue)

				newEnd := w.composing + utf8.RuneCountInString(tareaValue)
				w.w.SetComposingRegion(key.Range{Start: w.composing, End: newEnd})
				w.w.SetEditorSelection(key.Range{Start: newEnd, End: newEnd})
			} else {
				replaceRange := key.Range{Start: absStart, End: absEnd}
				w.w.EditorReplace(replaceRange, "")
				w.w.SetEditorSelection(key.Range{Start: absStart, End: absStart})
			}

		case "insertReplacementText":
			if w.composing != -1 {
				// During composition, replace the entire composition.
				compEnd := w.composing + utf8.RuneCountInString(data)
				replaceRange := key.Range{Start: w.composing, End: compEnd}
				w.w.EditorReplace(replaceRange, data)

				newEnd := w.composing + utf8.RuneCountInString(data)
				w.w.SetComposingRegion(key.Range{Start: -1, End: -1})
				w.w.SetEditorSelection(key.Range{Start: newEnd, End: newEnd})
				w.composing = -1
				w.lastCursor = newEnd
			} else {
				// Safari sends "insertReplacementText" for autocorrect, but the cursor is at the end of the word, so we need to find the word start.
				insertLen := utf8.RuneCountInString(data)
				wordStart := absStart

				if absStart > snippetStart {
					relPos := absStart - snippetStart
					snippetRunes := []rune(st.Snippet.Text)

					for i := relPos - 1; i >= 0; i-- {
						if i >= len(snippetRunes) {
							continue
						}
						r := snippetRunes[i]
						if r == ' ' || r == '\t' || r == '\n' || r == '\r' {
							break
						}
						wordStart = snippetStart + i
					}
				}

				replaceRange := key.Range{Start: wordStart, End: absStart}
				w.w.EditorReplace(replaceRange, data)

				newCursor := wordStart + insertLen
				w.w.SetEditorSelection(key.Range{Start: newCursor, End: newCursor})
				w.lastCursor = newCursor
			}

		case "insertText":
			if w.composing != -1 {
				compEnd := w.composing + utf8.RuneCountInString(data)
				replaceRange := key.Range{Start: w.composing, End: compEnd}
				w.w.EditorReplace(replaceRange, data)

				newEnd := w.composing + utf8.RuneCountInString(data)
				w.w.SetComposingRegion(key.Range{Start: -1, End: -1})
				w.w.SetEditorSelection(key.Range{Start: newEnd, End: newEnd})
				w.composing = -1
				w.lastCursor = newEnd
			} else {
				insertLen := utf8.RuneCountInString(data)
				replaceRange := key.Range{Start: absStart, End: absStart}
				if absStart != absEnd {
					replaceRange = key.Range{Start: absStart, End: absEnd}
				}

				newCursor := replaceRange.Start + insertLen
				w.w.EditorReplace(replaceRange, data)
				w.w.SetEditorSelection(key.Range{Start: newCursor, End: newCursor})
				w.lastCursor = newCursor
			}

		default: // paste and other input types
			if w.composing != -1 {
				compEnd := w.composing + utf8.RuneCountInString(tareaValue)
				replaceRange := key.Range{Start: w.composing, End: compEnd}
				w.w.EditorReplace(replaceRange, tareaValue)

				newEnd := w.composing + utf8.RuneCountInString(tareaValue)
				w.w.SetComposingRegion(key.Range{Start: w.composing, End: newEnd})
				w.w.SetEditorSelection(key.Range{Start: newEnd, End: newEnd})
			} else {
				replaceRange := key.Range{Start: absStart, End: absEnd}
				w.w.EditorReplace(replaceRange, tareaValue)

				newCursor := absStart + utf8.RuneCountInString(tareaValue)
				w.w.SetEditorSelection(key.Range{Start: newCursor, End: newCursor})
			}
		}

		return nil
	})
	w.addEventListener(w.tarea, "paste", func(this js.Value, args []js.Value) interface{} {
		if w.clipboard.IsUndefined() {
			return nil
		}
		// Prevents duplicated-paste, since "paste" is already handled through Clipboard API.
		args[0].Call("preventDefault")
		return nil
	})
}

func (w *window) addHistory() {
	w.browserHistory.Call("pushState", nil, nil, w.window.Get("location").Get("href"))
}

func (w *window) blur() {
	w.tarea.Call("blur")
	w.requestFocus = false
}

func (w *window) focus() {
	w.tarea.Call("focus")
	w.requestFocus = true
}

func (w *window) keyboard(hint key.InputHint) {
	var m string
	switch hint {
	case key.HintAny:
		m = "text"
	case key.HintText:
		m = "text"
	case key.HintNumeric:
		m = "decimal"
	case key.HintEmail:
		m = "email"
	case key.HintURL:
		m = "url"
	case key.HintTelephone:
		m = "tel"
	case key.HintPassword:
		m = "password"
	default:
		m = "text"
	}
	w.tarea.Set("inputMode", m)

	// Update autocomplete / autocorrect attributes.
	var autocomplete, autocorrect, autocapitalize string
	var spellcheck bool

	switch hint {
	case key.HintAny, key.HintText:
		autocomplete, autocorrect, autocapitalize, spellcheck = "on", "on", "on", true
	case key.HintEmail:
		autocomplete, autocorrect, autocapitalize, spellcheck = "email", "off", "off", false
	case key.HintURL:
		autocomplete, autocorrect, autocapitalize, spellcheck = "url", "off", "off", false
	case key.HintTelephone:
		autocomplete, autocorrect, autocapitalize, spellcheck = "tel", "off", "off", false
	case key.HintPassword:
		autocomplete, autocorrect, autocapitalize, spellcheck = "current-password", "off", "off", false
	default: // key.HintNumeric and others
		autocomplete, autocorrect, autocapitalize, spellcheck = "off", "off", "off", false
	}

	w.tarea.Set("autocomplete", autocomplete)
	w.tarea.Set("autocorrect", autocorrect)
	w.tarea.Set("autocapitalize", autocapitalize)
	w.tarea.Set("spellcheck", spellcheck)
}

func (w *window) keyEvent(e js.Value, ks key.State) {
	k := e.Get("key").String()

	if n, ok := translateKey(k); ok {
		if ks == key.Press {
			isMod := n == key.NameAlt || n == key.NameCommand || n == key.NameCtrl || n == key.NameShift || n == key.NameSuper
			isFunc := n == key.NameUpArrow || n == key.NameDownArrow || n == key.NameLeftArrow || n == key.NameRightArrow ||
				n == key.NamePageUp || n == key.NamePageDown || n == key.NameHome || n == key.NameEnd ||
				n == key.NameEscape || n == key.NameReturn || n == key.NameEnter || n == key.NameTab ||
				n == key.NameDeleteBackward || n == key.NameDeleteForward

			if isMod || isFunc {
				// Gio will request the browser to change the selection/carret position natively.
				e.Call("preventDefault")
			}
		}

		cmd := key.Event{
			Name:      n,
			Modifiers: modifiersFor(e),
			State:     ks,
		}
		w.processEvent(cmd)
	}
}

func (w *window) ProcessEvent(e event.Event) {
	w.processEvent(e)
}

func (w *window) processEvent(e event.Event) bool {
	if !w.w.ProcessEvent(e) {
		return false
	}
	select {
	case w.wakeups <- struct{}{}:
	default:
	}
	return true
}

func (w *window) Event() event.Event {
	for {
		evt, ok := w.w.nextEvent()
		if ok {
			if _, destroy := evt.(DestroyEvent); destroy {
				w.cleanup()
			}
			return evt
		}
		<-w.wakeups
	}
}

func (w *window) Invalidate() {
	w.w.Invalidate()
}

func (w *window) Run(f func()) {
	f()
}

func (w *window) Frame(frame *op.Ops) {
	w.w.ProcessFrame(frame, nil)
}

// modifiersFor returns the modifier set for a DOM MouseEvent or
// KeyEvent.
func modifiersFor(e js.Value) key.Modifiers {
	var mods key.Modifiers
	if e.Get("getModifierState").IsUndefined() {
		// Some browsers doesn't support getModifierState.
		return mods
	}
	if e.Call("getModifierState", "Alt").Bool() {
		mods |= key.ModAlt
	}
	if e.Call("getModifierState", "Control").Bool() {
		mods |= key.ModCtrl
	}
	if e.Call("getModifierState", "Shift").Bool() {
		mods |= key.ModShift
	}
	if e.Call("getModifierState", "Meta").Bool() {
		mods |= key.ModCommand
	}
	if e.Call("getModifierState", "OS").Bool() {
		mods |= key.ModSuper
	}
	return mods
}

func (w *window) touchEvent(kind pointer.Kind, e js.Value) {
	e.Call("preventDefault")
	t := time.Duration(e.Get("timeStamp").Int()) * time.Millisecond
	changedTouches := e.Get("changedTouches")
	n := changedTouches.Length()
	rect := w.cnv.Call("getBoundingClientRect")
	scale := w.scale
	var mods key.Modifiers
	if e.Get("shiftKey").Bool() {
		mods |= key.ModShift
	}
	if e.Get("altKey").Bool() {
		mods |= key.ModAlt
	}
	if e.Get("ctrlKey").Bool() {
		mods |= key.ModCtrl
	}
	if e.Get("metaKey").Bool() {
		mods |= key.ModCommand
	}
	for i := 0; i < n; i++ {
		touch := changedTouches.Index(i)
		pid := w.touchIDFor(touch)
		x, y := touch.Get("clientX").Float(), touch.Get("clientY").Float()
		x -= rect.Get("left").Float()
		y -= rect.Get("top").Float()
		pos := f32.Point{
			X: float32(x) * scale,
			Y: float32(y) * scale,
		}
		w.processEvent(pointer.Event{
			Kind:      kind,
			Source:    pointer.Touch,
			Position:  pos,
			PointerID: pid,
			Time:      t,
			Modifiers: mods,
		})
	}
}

func (w *window) touchIDFor(touch js.Value) pointer.ID {
	id := touch.Get("identifier")
	for i, id2 := range w.touches {
		if id2.Equal(id) {
			return pointer.ID(i)
		}
	}
	pid := pointer.ID(len(w.touches))
	w.touches = append(w.touches, id)
	return pid
}

func (w *window) pointerEvent(kind pointer.Kind, dx, dy float32, e js.Value) {
	e.Call("preventDefault")
	x, y := e.Get("clientX").Float(), e.Get("clientY").Float()
	rect := w.cnv.Call("getBoundingClientRect")
	x -= rect.Get("left").Float()
	y -= rect.Get("top").Float()
	scale := w.scale
	pos := f32.Point{
		X: float32(x) * scale,
		Y: float32(y) * scale,
	}
	scroll := f32.Point{
		X: dx * scale,
		Y: dy * scale,
	}
	t := time.Duration(e.Get("timeStamp").Int()) * time.Millisecond
	jbtns := e.Get("buttons").Int()
	var btns pointer.Buttons
	if jbtns&1 != 0 {
		btns |= pointer.ButtonPrimary
	}
	if jbtns&2 != 0 {
		btns |= pointer.ButtonSecondary
	}
	if jbtns&4 != 0 {
		btns |= pointer.ButtonTertiary
	}
	w.processEvent(pointer.Event{
		Kind:      kind,
		Source:    pointer.Mouse,
		Buttons:   btns,
		Position:  pos,
		Scroll:    scroll,
		Time:      t,
		Modifiers: modifiersFor(e),
	})
}

func (w *window) addEventListener(this js.Value, event string, f func(this js.Value, args []js.Value) interface{}) {
	jsf := w.funcOf(f)
	this.Call("addEventListener", event, jsf)
	w.cleanfuncs = append(w.cleanfuncs, func() {
		this.Call("removeEventListener", event, jsf)
	})
}

// funcOf is like js.FuncOf but adds the js.Func to a list of
// functions to be released during cleanup.
func (w *window) funcOf(f func(this js.Value, args []js.Value) interface{}) js.Func {
	jsf := js.FuncOf(f)
	w.cleanfuncs = append(w.cleanfuncs, jsf.Release)
	return jsf
}

func (w *window) EditorStateChanged(old, new editorState) {
	if w.composing != -1 {
		// Do not interfere with browser state while composing.
		// On Javascript is not possible to cancel the composition once started!
		return
	}

	// Update textarea value to match the snippet.
	if old.Snippet != new.Snippet {
		w.tarea.Set("value", new.Snippet.Text)
	}

	// Update selection to match Gio's selection.
	if old.Selection.Range != new.Selection.Range || old.Snippet != new.Snippet {
		if new.Selection.Range.Start != -1 && new.Selection.Range.End != -1 {
			// Calculate selection positions relative to snippet start.
			// The textarea contains only the snippet text.
			snippetStart := new.Snippet.Range.Start
			snippetEnd := new.Snippet.Range.End

			selStart := new.Selection.Range.Start
			selEnd := new.Selection.Range.End
			if selStart < snippetStart {
				selStart = snippetStart
			}
			if selStart > snippetEnd {
				selStart = snippetEnd
			}
			if selEnd < snippetStart {
				selEnd = snippetStart
			}
			if selEnd > snippetEnd {
				selEnd = snippetEnd
			}

			// Convert absolute rune positions to UTF-16 positions for the textarea.
			startUTF16 := new.UTF16Index(selStart)
			endUTF16 := new.UTF16Index(selEnd)

			// Convert to snippet-relative UTF-16 positions.
			snippetStartUTF16 := new.UTF16Index(snippetStart)
			start := startUTF16 - snippetStartUTF16
			end := endUTF16 - snippetStartUTF16

			if start < 0 {
				start = 0
			}
			if end < 0 {
				end = 0
			}

			// Calculate max UTF-16 length of snippet text.
			textLen := new.UTF16Index(snippetEnd) - snippetStartUTF16
			if start > textLen {
				start = textLen
			}
			if end > textLen {
				end = textLen
			}

			if start > end {
				start, end = end, start
			}

			w.tarea.Set("selectionStart", start)
			w.tarea.Set("selectionEnd", end)
		}
	}

	// Move DOM element to position the caret.
	if old.Selection.Caret != new.Selection.Caret || old.Selection.Transform != new.Selection.Transform {
		pos := new.Selection.Transform.Transform(new.Selection.Caret.Pos.Add(f32.Pt(0, new.Selection.Caret.Descent)))
		bounds := w.cnv.Call("getBoundingClientRect")
		left := bounds.Get("left").Float() + float64(pos.X)/float64(w.scale)
		top := bounds.Get("top").Float() + float64(pos.Y-new.Selection.Caret.Ascent)/float64(w.scale)
		height := float64(new.Selection.Caret.Ascent+new.Selection.Caret.Descent) / float64(w.scale)

		style := w.tarea.Get("style")
		style.Set("left", fmt.Sprintf("%fpx", left))
		style.Set("top", fmt.Sprintf("%fpx", top))
		style.Set("height", fmt.Sprintf("%fpx", height))
		style.Set("width", "1px")
	}
}

func (w *window) SetAnimating(anim bool) {
	w.animating = anim
	if anim && !w.animRequested {
		w.animRequested = true
		w.requestAnimationFrame.Invoke(w.redraw)
	}
}

func (w *window) ReadClipboard() {
	if w.clipboard.IsUndefined() {
		return
	}
	if w.clipboard.Get("readText").Truthy() {
		w.clipboard.Call("readText").Call("then", w.clipboardCallback)
	}
}

func (w *window) WriteClipboard(mime string, s []byte) {
	if w.clipboard.IsUndefined() {
		return
	}
	if w.clipboard.Get("writeText").IsUndefined() {
		return
	}
	w.clipboard.Call("writeText", string(s))
}

func (w *window) Configure(options []Option) {
	prev := w.config
	cnf := w.config
	cnf.apply(unit.Metric{}, options)
	// Decorations are never disabled.
	cnf.Decorated = true

	if prev.Title != cnf.Title {
		w.config.Title = cnf.Title
		w.document.Set("title", cnf.Title)
	}
	if prev.Mode != cnf.Mode {
		w.windowMode(cnf.Mode)
	}
	if prev.NavigationColor != cnf.NavigationColor {
		w.config.NavigationColor = cnf.NavigationColor
		w.navigationColor(cnf.NavigationColor)
	}
	if prev.Orientation != cnf.Orientation {
		w.config.Orientation = cnf.Orientation
		w.orientation(cnf.Orientation)
	}
	if cnf.Decorated != prev.Decorated {
		w.config.Decorated = cnf.Decorated
	}
	w.processEvent(ConfigEvent{Config: w.config})
}

func (w *window) Perform(system.Action) {}

var webCursor = [...]string{
	pointer.CursorDefault:                  "default",
	pointer.CursorNone:                     "none",
	pointer.CursorText:                     "text",
	pointer.CursorVerticalText:             "vertical-text",
	pointer.CursorPointer:                  "pointer",
	pointer.CursorCrosshair:                "crosshair",
	pointer.CursorAllScroll:                "all-scroll",
	pointer.CursorColResize:                "col-resize",
	pointer.CursorRowResize:                "row-resize",
	pointer.CursorGrab:                     "grab",
	pointer.CursorGrabbing:                 "grabbing",
	pointer.CursorNotAllowed:               "not-allowed",
	pointer.CursorWait:                     "wait",
	pointer.CursorProgress:                 "progress",
	pointer.CursorNorthWestResize:          "nw-resize",
	pointer.CursorNorthEastResize:          "ne-resize",
	pointer.CursorSouthWestResize:          "sw-resize",
	pointer.CursorSouthEastResize:          "se-resize",
	pointer.CursorNorthSouthResize:         "ns-resize",
	pointer.CursorEastWestResize:           "ew-resize",
	pointer.CursorWestResize:               "w-resize",
	pointer.CursorEastResize:               "e-resize",
	pointer.CursorNorthResize:              "n-resize",
	pointer.CursorSouthResize:              "s-resize",
	pointer.CursorNorthEastSouthWestResize: "nesw-resize",
	pointer.CursorNorthWestSouthEastResize: "nwse-resize",
}

func (w *window) SetCursor(cursor pointer.Cursor) {
	style := w.cnv.Get("style")
	style.Set("cursor", webCursor[cursor])
}

func (w *window) ShowTextInput(show bool) {
	// Run in a goroutine to avoid a deadlock if the
	// focus change result in an event.
	if show {
		w.focus()
	} else {
		// If we're composing, end composition first by clearing the textarea.
		// That is a attempt to force the browser to end composition.
		if w.composing != -1 {
			w.tarea.Set("value", "")
			w.composing = -1
			w.w.SetComposingRegion(key.Range{Start: -1, End: -1})
		}
		w.blur()
	}
}

func (w *window) SetInputHint(mode key.InputHint) {
	w.keyboard(mode)
}

func (w *window) resize() {
	w.scale = float32(w.window.Get("devicePixelRatio").Float())

	rect := w.cnv.Call("getBoundingClientRect")
	size := image.Point{
		X: int(float32(rect.Get("width").Float()) * w.scale),
		Y: int(float32(rect.Get("height").Float()) * w.scale),
	}
	if size != w.config.Size {
		w.config.Size = size
		w.processEvent(ConfigEvent{Config: w.config})
	}

	if vx, vy := w.visualViewport.Get("width"), w.visualViewport.Get("height"); !vx.IsUndefined() && !vy.IsUndefined() {
		w.inset.X = float32(w.config.Size.X) - float32(vx.Float())*w.scale
		w.inset.Y = float32(w.config.Size.Y) - float32(vy.Float())*w.scale
	}

	if w.config.Size.X == 0 || w.config.Size.Y == 0 {
		return
	}

	w.cnv.Set("width", w.config.Size.X)
	w.cnv.Set("height", w.config.Size.Y)
}

func (w *window) draw(sync bool) {
	if w.contextStatus == contextStatusLost {
		return
	}
	anim := w.animating
	w.animRequested = anim
	if anim {
		w.requestAnimationFrame.Invoke(w.redraw)
	} else if !sync {
		return
	}
	size, insets, metric := w.getConfig()
	if metric == (unit.Metric{}) || size.X == 0 || size.Y == 0 {
		return
	}

	w.processEvent(frameEvent{
		FrameEvent: FrameEvent{
			Now:    time.Now(),
			Size:   size,
			Insets: insets,
			Metric: metric,
		},
		Sync: sync,
	})
}

func (w *window) getConfig() (image.Point, Insets, unit.Metric) {
	invscale := unit.Dp(1. / w.scale)
	return image.Pt(w.config.Size.X, w.config.Size.Y),
		Insets{
			Bottom: unit.Dp(w.inset.Y) * invscale,
			Right:  unit.Dp(w.inset.X) * invscale,
		}, unit.Metric{
			PxPerDp: w.scale,
			PxPerSp: w.scale,
		}
}

func (w *window) windowMode(mode WindowMode) {
	switch mode {
	case Windowed:
		if !w.document.Get("fullscreenElement").Truthy() {
			return // Browser is already Windowed.
		}
		if !w.document.Get("exitFullscreen").Truthy() {
			return // Browser doesn't support such feature.
		}
		w.document.Call("exitFullscreen")
		w.config.Mode = Windowed
	case Fullscreen:
		elem := w.document.Get("documentElement")
		if !elem.Get("requestFullscreen").Truthy() {
			return // Browser doesn't support such feature.
		}
		elem.Call("requestFullscreen")
		w.config.Mode = Fullscreen
	}
}

func (w *window) orientation(mode Orientation) {
	if j := w.screenOrientation; !j.Truthy() || !j.Get("unlock").Truthy() || !j.Get("lock").Truthy() {
		return // Browser don't support Screen Orientation API.
	}

	switch mode {
	case AnyOrientation:
		w.screenOrientation.Call("unlock")
	case LandscapeOrientation:
		w.screenOrientation.Call("lock", "landscape").Call("then", w.redraw)
	case PortraitOrientation:
		w.screenOrientation.Call("lock", "portrait").Call("then", w.redraw)
	}
}

func (w *window) navigationColor(c color.NRGBA) {
	theme := w.head.Call("querySelector", `meta[name="theme-color"]`)
	if !theme.Truthy() {
		theme = w.document.Call("createElement", "meta")
		theme.Set("name", "theme-color")
		w.head.Call("appendChild", theme)
	}
	rgba := f32color.NRGBAToRGBA(c)
	theme.Set("content", fmt.Sprintf("#%06X", []uint8{rgba.R, rgba.G, rgba.B}))
}

func osMain() {
	select {}
}

func translateKey(k string) (key.Name, bool) {
	var n key.Name

	switch k {
	case "ArrowUp":
		n = key.NameUpArrow
	case "ArrowDown":
		n = key.NameDownArrow
	case "ArrowLeft":
		n = key.NameLeftArrow
	case "ArrowRight":
		n = key.NameRightArrow
	case "Escape":
		n = key.NameEscape
	case "Enter":
		n = key.NameReturn
	case "Backspace":
		n = key.NameDeleteBackward
	case "Delete":
		n = key.NameDeleteForward
	case "Home":
		n = key.NameHome
	case "End":
		n = key.NameEnd
	case "PageUp":
		n = key.NamePageUp
	case "PageDown":
		n = key.NamePageDown
	case "Tab":
		n = key.NameTab
	case " ":
		n = key.NameSpace
	case "F1":
		n = key.NameF1
	case "F2":
		n = key.NameF2
	case "F3":
		n = key.NameF3
	case "F4":
		n = key.NameF4
	case "F5":
		n = key.NameF5
	case "F6":
		n = key.NameF6
	case "F7":
		n = key.NameF7
	case "F8":
		n = key.NameF8
	case "F9":
		n = key.NameF9
	case "F10":
		n = key.NameF10
	case "F11":
		n = key.NameF11
	case "F12":
		n = key.NameF12
	case "Control":
		n = key.NameCtrl
	case "Shift":
		n = key.NameShift
	case "Alt":
		n = key.NameAlt
	case "OS":
		n = key.NameSuper
	default:
		r, s := utf8.DecodeRuneInString(k)
		// If there is exactly one printable character, return that.
		if s == len(k) && unicode.IsPrint(r) {
			return key.Name(strings.ToUpper(k)), true
		}
		return "", false
	}
	return n, true
}

func (JSViewEvent) implementsViewEvent() {}
func (JSViewEvent) ImplementsEvent()     {}
func (j JSViewEvent) Valid() bool {
	return !(j.Element.IsNull() || j.Element.IsUndefined())
}