[DevTools] Account for synthetic events in deepElementFromPoint()

third_party/WebKit/Source/devtools/front_end/ui/GlassPane.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 UI.GlassPane = class {
   constructor() {
     this._widget = new UI.Widget(true);
     this._widget.markAsRoot();
     this.element = this._widget.element;
     this.contentElement = this._widget.contentElement;
@@ skipping 125 matching lines @@
     this.element.ownerDocument.body.removeEventListener('click', this._onMouseDownBound, true);
     this._widget.detach();
   }
 
   /**
    * @param {!Event} event
    */
   _onMouseDown(event) {
     if (!this._onClickOutsideCallback)
       return;
-    if (this.contentElement.isSelfOrAncestor(/** @type {?Node} */ (event.deepElementFromPoint())))
+    var node = event.deepElementFromPoint();
+    if (!node || this.contentElement.isSelfOrAncestor(node))
       return;
     this._onClickOutsideCallback.call(null, event);
   }
 
   _positionContent() {
     if (!this.isShowing())
       return;
 
     var showArrow = this._marginBehavior === UI.GlassPane.MarginBehavior.Arrow;
     var gutterSize = showArrow ? 8 : (this._marginBehavior === UI.GlassPane.MarginBehavior.NoMargin ? 0 : 3);
@@ skipping 231 matching lines @@
 UI.GlassPane.MarginBehavior = {
   Arrow: Symbol('Arrow'),
   DefaultMargin: Symbol('DefaultMargin'),
   NoMargin: Symbol('NoMargin')
 };
 
 /** @type {!Map<!Document, !Element>} */
 UI.GlassPane._containers = new Map();
 /** @type {!Set<!UI.GlassPane>} */
 UI.GlassPane._panes = new Set();