[MD settings] shrink overscroll when scrolling up

chrome/browser/resources/settings/settings_main/settings_main.js

Index: chrome/browser/resources/settings/settings_main/settings_main.js
diff --git a/chrome/browser/resources/settings/settings_main/settings_main.js b/chrome/browser/resources/settings/settings_main/settings_main.js
index 6aa8c1113f14754117d1c1a2bfb7fba3bd9b450b..da91aeab2701f29da46f2187efcbce28c74afcdf 100644
--- a/chrome/browser/resources/settings/settings_main/settings_main.js
+++ b/chrome/browser/resources/settings/settings_main/settings_main.js
@@ -78,6 +78,21 @@ Polymer({
       settings.navigateTo(this.advancedToggleExpanded_ ?
           settings.Route.ADVANCED : settings.Route.BASIC);
     }.bind(this));
+
+    var mainContainer = this.domHost.$$('paper-header-panel').$.mainContainer;

[Dan Beam, 2016/08/03 01:30:09]

can we not poke into paper-header-panel's local DOM?

[dschuyler, 2016/08/03 19:23:08]

I changed it to something a bit more direct.
I'm not sure how to get the scroll event and
data without looking at the #mainContainer.

Done.

+    var overscroll = this.$.overscroll;
+    mainContainer.addEventListener('scroll', function() {
+      // TODO(dschuyler): This works okay, but not perfectly with page-up/down
+      // and home/end. It may have to do with the order in which the resize
+      // occurs (relative to the scroll action).
+      var visibleBottom = mainContainer.scrollTop + mainContainer.clientHeight;
+      var overscrollBottom = overscroll.offsetTop + overscroll.scrollHeight;
+      // How much of the overscroll is visible (may be negative).
+      var visibleOverscroll = overscroll.scrollHeight -
+                              (overscrollBottom - visibleBottom);
+      if (visibleOverscroll > 0)

[Dan Beam, 2016/08/03 01:30:09]

so why are we only setting this when overscroll > 0?  shouldn't we just be doing
Math.min(0, visibleOverscroll)?

[dschuyler, 2016/08/03 19:23:08]

Done.

That would be logically the same, but function
calls have a high overhead compared to the 
conditional. Since it was logically the same 
and equally readable, I was going for the one 
with less overhead.

+        overscroll.style.paddingBottom = visibleOverscroll + 'px';
+    }.bind(this));
   },
 
   /**