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..4b6cd8580ab01aa3f22dfe1a3a20459bba15f923 100644
--- a/chrome/browser/resources/ntp4/md_incognito_tab.js
+++ b/chrome/browser/resources/ntp4/md_incognito_tab.js
@@ -24,8 +24,19 @@ 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
@@ -37,4 +48,9 @@ window.addEventListener('load', function() {
   }
 
   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.

 });