If a page has a div styled to show scrollbars when the contents exceed their pre-defined size (CSS overflow ), WebKit/GTK+ will not display scrollbars, but you will be able to scroll the contents with the mouse wheel. I'm attaching a very simple HTML that demonstrates the problem.
Created attachment 26969 [details]
simple test case
I tested this with Epiphany/Gecko, Epiphany/WebKit, Midori and Arora; Arora correctly (I believe) displays both scrollbars, Epiphany/Gecko displays only the vertical bar.
This is fixed in trunk now, see bug https://bugs.webkit.org/show_bug.cgi?id=23711.