Show confirmation dialog on leaving WebUI while composing (#5616)

* Show confirmation dialog on leaving WebUI while composing

Currently, Back button and Back hotkey can cause leaving from WebUI, as well as browser's back button. Users may hit those buttons accidentally, and their composing text will be lost.

So this prevents it by showing confirmation dialog from `onbeforeunload` event.

* Fix message and comments

app/javascript/mastodon/features/ui/index.js

@@ -39,13 +39,19 @@ import {
 } from './util/async-components';
 import { HotKeys } from 'react-hotkeys';
 import { me } from '../../initial_state';
+import { defineMessages, injectIntl } from 'react-intl';
 
 // Dummy import, to make sure that <Status /> ends up in the application bundle.
 // Without this it ends up in ~8 very commonly used bundles.
 import '../../components/status';
 
+const messages = defineMessages({
+  beforeUnload: { id: 'ui.beforeunload', defaultMessage: 'Your draft will be lost if you leave Mastodon.' },
+});
+
 const mapStateToProps = state => ({
   isComposing: state.getIn(['compose', 'is_composing']),
+  hasComposingText: state.getIn(['compose', 'text']) !== '',
 });
 
 const keyMap = {
@@ -75,6 +81,7 @@ const keyMap = {
 };
 
 @connect(mapStateToProps)
+@injectIntl
 @withRouter
 export default class UI extends React.Component {
 
@@ -86,7 +93,9 @@ export default class UI extends React.Component {
     dispatch: PropTypes.func.isRequired,
     children: PropTypes.node,
     isComposing: PropTypes.bool,
+    hasComposingText: PropTypes.bool,
     location: PropTypes.object,
+    intl: PropTypes.object.isRequired,
   };
 
   state = {
@@ -94,6 +103,17 @@ export default class UI extends React.Component {
     draggingOver: false,
   };
 
+  handleBeforeUnload = (e) => {
+    const { intl, isComposing, hasComposingText } = this.props;
+
+    if (isComposing && hasComposingText) {
+      // Setting returnValue to any string causes confirmation dialog.
+      // Many browsers no longer display this text to users,
+      // but we set user-friendly message for other browsers, e.g. Edge.
+      e.returnValue = intl.formatMessage(messages.beforeUnload);
+    }
+  }
+
   handleResize = debounce(() => {
     // The cached heights are no longer accurate, invalidate
     this.props.dispatch(clearHeight());
@@ -168,6 +188,7 @@ export default class UI extends React.Component {
   }
 
   componentWillMount () {
+    window.addEventListener('beforeunload', this.handleBeforeUnload, false);
     window.addEventListener('resize', this.handleResize, { passive: true });
     document.addEventListener('dragenter', this.handleDragEnter, false);
     document.addEventListener('dragover', this.handleDragOver, false);
@@ -209,6 +230,7 @@ export default class UI extends React.Component {
   }
 
   componentWillUnmount () {
+    window.removeEventListener('beforeunload', this.handleBeforeUnload);
     window.removeEventListener('resize', this.handleResize);
     document.removeEventListener('dragenter', this.handleDragEnter);
     document.removeEventListener('dragover', this.handleDragOver);