MD Settings: Workaround for section expand/collapse animation jank.

chrome/browser/resources/settings/settings_page_css.html

Index: chrome/browser/resources/settings/settings_page_css.html
diff --git a/chrome/browser/resources/settings/settings_page_css.html b/chrome/browser/resources/settings/settings_page_css.html
index 1b6d6b73d0f2c413b0a7e91d3d058dc0e98fbb87..d6f108936df5f4328096c33bd16384ddfb7aef8f 100644
--- a/chrome/browser/resources/settings/settings_page_css.html
+++ b/chrome/browser/resources/settings/settings_page_css.html
@@ -25,11 +25,22 @@
          * with the top item in the side nav menu. Both things are desired
          * by Alan (bettes@). */
         margin-bottom: 3px;
-        padding-top: var(--settings-page-padding-top);
+        /* TODO(michaelpg): This makes more sense as padding, but uses margin as
+         * a workaround for crbug.com/617287. Because margins collapse, add 3px
+         * to retain mostly the same spacing. */
+        margin-top: calc(var(--settings-page-padding-top) + 3px);
+      }
+
+      :host > div > :first-of-type {
+        /* TODO(michaelpg): This is offset because of the previous addition of
+         * 3px. */

[dschuyler, 2016/06/08 01:04:41]

Let's include the bug number at 
each point of change, so that 
when the bug is eventually fixed
we can grep and find all of them.
e.g.
/* TODO(michaelpg): This is offset because of the previous addition of
 * 3px. crbug.com/617287. */

[michaelpg, 2016/06/09 18:31:30]

Done.

+        margin-top: var(--settings-page-padding-top);
       }
 
       .expanded.frozen {
-        margin-top: -var(--settings-page-padding-top);
+        /* TODO(michaelpg): This is zeroed out because of the previous
+         * replacement of margin with padding, above. */

[dschuyler, 2016/06/08 01:04:41]

 * replacement of margin with padding, above. crbug.com/617287. */

[michaelpg, 2016/06/09 18:31:30]

Done.

+        margin-top: 0;  /* -var(--settings-page-padding-top); */
       }
     </style>
   </template>