WebView keeps track of whether its HWND is hidden or shown so that it can stop painting while it's hidden. But it doesn't keep track of whether any of its ancestors' HWNDs are hidden, so we unnecessarily paint when WebView itself is not hidden but one of its ancestors is. There doesn't seem to be any notification sent to WebView when one of its ancestors becomes hidden (similar to -[NSView viewDidHide]), so I'm not sure how we would fix this.
<rdar://problem/8977301>