[Webkit-unassigned] [Bug 193192] New: [css-sticky] sticky canvas obscures scrollbars of (flexbox?) parent

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=193192

            Bug ID: 193192
           Summary: [css-sticky] sticky canvas obscures scrollbars of
                    (flexbox?) parent
           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: andyearnshaw at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 358488

  --> https://bugs.webkit.org/attachment.cgi?id=358488&action=review

Self-contained example

Open the attached HTML file. You will see a canvas element with a grid drawn on it. The canvas is a sticky-positioned child of a container, set to 100% width and height, with another element in the flex flow that sets an artificial scroll area for the canvas.

When you hover over the canvas and scroll, you cannot see the scrollbars on either axis until you reach the over-scroll point.  When the canvas is not sticky and is instead contained within a sticky div, the problem goes away.

NOTE: the only way I could achieve this kind of sticky positioning was to use a container with `display: flex`, as WebKit does not respect `position: sticky` on a parent that has `overflow: auto`.  Therefore, I'm unsure as to whether flexbox is part of the issue.

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