Created attachment 90700 [details]
With gtk2 you could scroll the page even when scrollbars was disabled, but not with gtk+-3.0
scrolling other gtkwidgets (such as textview) this way works with both gtk2 and gtk3, therefore I'm assuming this to be a webkit-gtk bug rather than a gtk+ bug. Attaching a modified gtklauncher that shows this behaviour and a small example doing about the same thing but with a textview.
Created attachment 90701 [details]