[Webkit-unassigned] [Bug 195845] New: UI-process hit-testing needs to know about containing block relationships

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=195845

            Bug ID: 195845
           Summary: UI-process hit-testing needs to know about containing
                    block relationships
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Scrolling
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: simon.fraser at apple.com

Created attachment 364917

  --> https://bugs.webkit.org/attachment.cgi?id=364917&action=review

Simple testcase

When we hit-test in the UI process, hit-testing via the UIView hierarchy isn't always correct, because that reflects painting order, not hit-testing order.

This matters when overflow:scroll contains a composited layer (say, position:relative) whose stacking context is outside the overflow. You can't scroll the overflow:scroll if you happen to hit this box.

This breaks scrolling at https://palace-games.com:

1. Load https://palace-games.com/ on iPad
2. Tap “Reserve a room today”
3. Try to scroll the list of rooms; try touching in different places. Sometimes the overflow doesn't scroll, and you rubber-band the body instead.

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