Fix offsetParent computation for elements with fixed position (#54043)

Consider this test case:

Observed behavior: div#fixed has offsetParent = null, but its offsetLeft and offsetTop are relative to div#transform (both values = 0).

Specification conflict, current CSSOM spec states:

The element’s computed value of the position property is fixed. This contradicts the observed behavior since transform establishes a containing block for fixed-position descendants. Other properties like will-change and contain can trigger this too.

Proposed Specification Update of offsetParent:

If any of the following holds true return null and terminate this algorithm:

• The element’s computed value of the position property is fixed and no ancestor establishes a fixed position containing block.

Let ancestor be the parent of the element in the flat tree and repeat these substeps:

• If ancestor is closed-shadow-hidden from the element, the element has a computed position value of fixed, and no ancestor establishes a fixed position containing block, terminate this algorithm and return null.

Implement it in Chromium first, then update the spec. See https://github.com/w3c/csswg-drafts/issues/12352 for more details.

Bug: 40694036 Change-Id: I4bf32a9a5ab73c44051d13fd9c79eac7515c973e Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6774502 Reviewed-by: Ian Kilpatrick ikilpatrick@chromium.org Commit-Queue: Peng Zhou zhoupeng.1996@bytedance.com Cr-Commit-Position: refs/heads/main@{#1492432}

Co-authored-by: zhoupeng zhoupeng.1996@bytedance.com