[Webkit-unassigned] [Bug 192877] New: user scrollable overflow causes behavior "as-if-positioned" lifting itself higher in paint order than if not overflowing

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=192877

            Bug ID: 192877
           Summary: user scrollable overflow causes behavior
                    "as-if-positioned" lifting itself higher in paint
                    order than if not overflowing
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: hi at jonjohnjohnson.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 357723

  --> https://bugs.webkit.org/attachment.cgi?id=357723&action=review

animating scrollable content to expose paint order bug

Can't seem to find a webkit bug tracking this, though it is seemingly known among vendor developers (https://github.com/w3c/csswg-drafts/issues/2717#issuecomment-395911418)

1. Open the attached case.
2. Watch overflow:auto box that has green content grow and shrink, creating and removing user scrollable content every 2 seconds.
3. See that when it overflows, it behaves as-if-positioned painted higher and lower in order than the red block (which also is always lifted higher because of overflow).

Compat
Firefox doesn't let "user scrollable" overflow affect paint order, red block is always behind green in test case.
Blink has same/similar issue, but is working on fix??? https://bugs.chromium.org/p/chromium/issues/detail?id=370604#c_ts1545253494


*I say user scrollable, because overflow:hidden doesn't create compositing order issue

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