* Go to https://secure.phabricator.com
* scroll around with arrow keys
* click on the address bar
* click back on the web content
* hit arrow keys, nothing happens.
Clicking "a bunch" (two?) times makes the scroll bars work again. Since (1) I ran into this all the time this morning (2) don't actually click on the address bar for fun, I think there are other variations that trigger this, but it's the cleanest repo case I could make.
This is with Firefox 31, which should be an ESR release. I get the same behavior with Chrome 34, but I know that one's older.