This happens with Qt 4.8.2/3 (no idea what Webkit version that is for above) If the caret location is *not* at the top of the screen on a QWebView where isContentEditable is set to True, and the user hits Page Down, then it scrolls an incorrect amount. It scrolls 1.y screens worth, where y is the distance from the top of the screen to the caret. Effectively you have to follow the page down with a page up to see the page worth of content that got missed. Page down only works correctly if the caret is at the very top of the visible window. Page up seems to be unaffected by this issue.
The Qt port has been removed from WebKit, resolving this bug.