[Webkit-unassigned] [Bug 240128] New: WebKit improperly overlaps a BFC with a float, with certain margin values

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=240128

            Bug ID: 240128
           Summary: WebKit improperly overlaps a BFC with a float, with
                    certain margin values
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dholbert at mozilla.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 458892

  --> https://bugs.webkit.org/attachment.cgi?id=458892&action=review

testcase 1

STR:
1. Load attached testcase.

EXPECTED RESULTS:
The blue area and the fuchsia area should **not be overlapping**.

ACTUAL RESULTS:
The blue bordered area overlaps the fuchsia area.

There are various pieces of CSS Float layout that are vague or undefined, but one clear requirement is (emphasis added by me):
> The border box of [...] an element in the normal flow that establishes a new block formatting context [...] **must not overlap the margin box of any floats in the same block formatting context as the element itself.** If necessary, implementations should clear the said element by placing it below any preceding floats, but may place it adjacent to such floats if there is sufficient space.

https://www.w3.org/TR/CSS21/visuren.html#floats

WebKit is violating this requirement here.

Chromium/Blink has the same bug, which is filed as https://bugs.chromium.org/p/chromium/issues/detail?id=1322774

Firefox gives EXPECTED RESULTS.

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