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 =
+      document.defaultView.getComputedStyle(document.body, null)

[Dan Beam, 2017/05/17 18:09:04]

i don't really see a difference between document.defaultView.getComputedStyle()
and window.getComputedStyle() in this case (which is shorter and more common). 
i also don't think you need the "null" arg

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

Done. Simplified, thanks.

+          .getPropertyValue("font-size");

[Dan Beam, 2017/05/17 18:09:04]

does zoom affect this value?

[msramek, 2017/05/18 15:16:28]

Good point :) I tested that neither the computed style font size nor the
offsetWidth are affected by zooming, so the caching should be valid.

+  var savedMaxWidth = window.localStorage.getItem(fontSize);

[Dan Beam, 2017/05/17 18:09:04]

can this all just be:

var fontSize = window.getComputedStyle(document.body).fontSize;
var maxWidth = localStorage[fontSize] ||
    bulletpoints[0].offsetWidth + bulletpoints[1].offsetWidth + 42;

?

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

Done. Yes, but I'd like to keep the comments to explain the constants.

+
+  if (savedMaxWidth) {
+    maxWidth = savedMaxWidth
+  } else {
+    var maxWidth = (bulletpoints[0].offsetWidth + bulletpoints[1].offsetWidth +
+        40 /* margin */ + 2 /* offsetWidths may be rounded down */);
+  }
 
   // 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";
+
+  // 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.
+  window.localStorage.setItem(fontSize, maxWidth);

[Dan Beam, 2017/05/17 18:09:04]

localStorage[fontSize] = maxWidth;

[msramek, 2017/05/18 15:16:28]

Done.

 });