[Webkit-unassigned] [Bug 117059] New: iOS touch scrolling doesn't take occluding elements into account

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=117059

           Summary: iOS touch scrolling doesn't take occluding elements
                    into account
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: HTML Events
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rbyers at chromium.org
                CC: eoconnor at apple.com


If you have a div using -webkit-overflow-scrolling: touch, then any touches in the region of that div will cause it to scroll, even if the touches are actually landing on another element that's on top of the scrolling element.  Repro here: http://jsbin.com/ezovup/2 - touching the header shouldn't scroll the underlying list (eg. it might have buttons you want to tap).

Now I don't know anything about how mobile Safari implements this (since their touch support is all in a fork of WebKit), but I assume it's similar to Chrome (except that we no longer rely on a special CSS property, accelerated touch scrolling is always enabled).  I assume that -webkit-overflow-scrolling: touch means that scrolling is handled off the main WebKit thread, and since only the main thread has access to the DOM this must rely on sending a list of 'fast scrollable regions' from the main thread.  So the bug is that the thread doing the scrolling knows about the rectangle for fast scrolling, but not about the rectangle of the div that's occluding it.

[discussion on G+ here: https://plus.google.com/115788095648461403871/posts/UXwZJbNtWGj]

Tested on iOS 6.1.3
Works properly on Chrome Android and ChromeOS

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