Speed up the Incognito NTP reload

chrome/browser/resources/ntp4/md_incognito_tab.js

Index: chrome/browser/resources/ntp4/md_incognito_tab.js
diff --git a/chrome/browser/resources/ntp4/md_incognito_tab.js b/chrome/browser/resources/ntp4/md_incognito_tab.js
index 3918f70126322fd2dc661c8f65c12be8a393ae16..a385f87d025b98662d2a71817aa5793e610c6a42 100644
--- a/chrome/browser/resources/ntp4/md_incognito_tab.js
+++ b/chrome/browser/resources/ntp4/md_incognito_tab.js
@@ -24,8 +24,18 @@ 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