[Webkit-unassigned] [Bug 229141] New: CSS transform interoperability of parent versus containing block for 3D Rendering Contexts and perspective

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=229141

            Bug ID: 229141
           Summary: CSS transform interoperability of parent versus
                    containing block for 3D Rendering Contexts and
                    perspective
           Product: WebKit
           Version: Safari 14
          Hardware: All
               URL: https://docs.google.com/document/d/1hPOMT0Uza9fxnSwAOW
                    x2W175VTdXBzRg1-706VBe4ik/edit
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dbaron at dbaron.org
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

I'm in the process of shipping some changes in Chromium that will mostly align with Gecko's behavior in a few ways:

 1. for handling of transform-style, make decisions about when to extend 3D Rendering Contexts based only on the DOM parent/child relationships, rather than considering containing block relationships (e.g., for absolute or fixed position, or for block-within-inline) or considering which nodes have transforms or not.

 2. for handling of the perspective property, apply the perspective only to DOM children, rather than considering the containing block hierarchy or applying to deeper descendants if the intermediate ones don't have certain transform properties set.

There's also a somewhat unrelated and pretty small change:

 3. Makes transform-style: preserve-3d cause an element to be a containing block for position: absolute and position: fixed elements even if a grouping property causes flattening of the 3D scene.

These changes are described in substantially more detail in https://docs.google.com/document/d/1hPOMT0Uza9fxnSwAOWx2W175VTdXBzRg1-706VBe4ik/edit , which also links to relevant tests in web-platform-tests.


I wanted to get a WebKit bug on file about these differences since we're now in a situation where Gecko and Chromium are becoming more compatible, but WebKit is different from them.  (That said, I'm aware there are a bunch of *other* differences in preserve-3d in WebKit that make this pretty confusing to test/understand, although I think the underlying behavior differences still apply here.)

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