Visit any WebKit Bugzilla bug that has a patch and click the "Details" button in the attachment view. That brings you to a page like https://bugs.webkit.org/attachment.cgi?id=297957&action=edit. Now scroll down in the frame displaying the text file. The scrollbar moves, but none of the contents are scrolled. It affects frames on GNOME Bugzilla too. It's broken with 2.14.2, but trunk in my GNOME JHBuild environment is not affected. Like with bug #166683, I wonder if this is a regression in some dependency of WebKit, because I never noticed this before today, but we haven't had a WebKit update in months.
No clue what happened here, but it was an Epiphany problem. I "solved" it by downgrading and upgrading my Epiphany package. Very strange as that means nothing should have changed....