Created attachment 460775 [details] Testcase When the content size matches the view size (or the iframe size), then we'll create a vertical scrollbar and trigger vertical scrolling when it's not necessary (when scrollbars take space, i.e. non-overlay scrollbars). Turn on always-on scrollbars. Load the attached testcase Size the window so that the gray rectangle just covers the web view. Reload; note how a vertical scrollbar appears. Now size the window from the bottom, making it slightly taller then shorter, back to the original size. Note how you can make the scrollbar go away.
This is about code in in `FrameView::adjustScrollbarsForLayout()` that forces on the vertical scrollbar for the first layout, but doesn't compute that it can remove it again and still fit the content. The state is unstable; some future layout can cause the scrollbar to disappear.
<rdar://problem/97108398>