Speed up the Incognito NTP reload

chrome/browser/resources/ntp4/md_incognito_tab.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.
 
 // Handle the bookmark bar and theme change requests from the C++ side.
 var ntp = {
   /** @param {string} attached */
   setBookmarkBarAttached: function(attached) {
     document.documentElement.setAttribute('bookmarkbarattached', attached);
   },
 
   /** @param {!{hasCustomBackground: boolean}} themeData */
   themeChanged: function(themeData) {
     document.documentElement.setAttribute('hascustombackground',
                                           themeData.hasCustomBackground);
     $('incognitothemecss').href =
         'chrome://theme/css/incognito_new_tab_theme.css?' + Date.now();
   },
 };
 
 // Let the width of two lists of bulletpoints in a horizontal alignment
 // determine the maximum content width.
 window.addEventListener('load', function() {
   var bulletpoints = document.querySelectorAll('.bulletpoints');
   var content = document.querySelector('.content');
 
-  var maxWidth = (bulletpoints[0].offsetWidth + bulletpoints[1].offsetWidth +
-      40 /* margin */ + 2 /* offsetWidths may be rounded down */);
+  // Unless this is the first load of the Incognito NTP in this session and
+  // with this font size, we already have the maximum content width determined.
+  var fontSize =
+      window.getComputedStyle(document.body).getPropertyValue("font-size");

[Dan Beam, 2017/05/19 01:09:32]

don't really get why you prefer getPropertyValue('font-size') over .fontSize,
but if you do stay with this (for some reason), you shouldn't be using
double-quotes

[msramek, 2017/05/19 09:50:59]

Fixed. Sorry, I missed that in your original comment. I wrote this while reading
a documentation which suggests getPropertyValue() probably for cross-browser
reasons.

+  var maxWidth = localStorage[fontSize] ||
+      (bulletpoints[0].offsetWidth + bulletpoints[1].offsetWidth +
+           40 /* margin */ + 2 /* offsetWidths may be rounded down */);
+
+  // Save the data for quicker access when the NTP is reloaded. Note that since
+  // we're in the Incognito mode, the local storage is ephemeral and the data
+  // will be discarded when the session ends.
+  localStorage[fontSize] = maxWidth;

[msramek, 2017/05/18 15:16:29]

I fixed the ordering here. We want to save the result to the local storage
*before* capping it at 600 (line 44), so that when we restore it from the
localStorage, we can again apply the class 'tooWide' (line 46).

 
   // Limit the maximum width to 600px. That might force the two lists
   // of bulletpoints under each other, in which case we must swap the left
   // and right margin.
   if (maxWidth > 600) {
     maxWidth = 600;
 
     bulletpoints[1].classList.add('tooWide');
   }
 
   content.style.maxWidth = maxWidth + "px";
 });