The scrollbar position can become un-synchronized with the element it represents. This happens when an element has the "overflow:scroll" style and the size of its is increased when the scrollbar is already at the bottom of the scroll area. The scrollbar will remain at the bottom of the scroll area, while the actual scroll position of the document remains at the same location it was before.
Created attachment 14657 [details] Test case reduction Instructions: 1. Click the 'add line' button a couple of times until a vertical scrollbar appears. 2. Scroll to the bottom. 3.Click the 'add line' button a couple more times. Expected: 1. The scrollbar should remain synchronized with the document. Actual: 1. The scrollbar is positioned at the bottom, but the document remains positioned at the original location.
This looks like it may be related to bug 7686.
The reduction works fine for me on ToT WebKit.