[Webkit-unassigned] [Bug 243167] New: Tables render extra collapsed borders with text-based vertical alignment when cells contain block-level elements

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=243167

            Bug ID: 243167
           Summary: Tables render extra collapsed borders with text-based
                    vertical alignment when cells contain block-level
                    elements
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: macOS 12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Tables
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jordan at cryptica.me
                CC: zalan at apple.com

Created attachment 461197

  --> https://bugs.webkit.org/attachment.cgi?id=461197&action=review

Examples of correct and buggy behavior

I checked this bug against Safari 15 *and* the latest Tech Preview.

When you have a table that has cells that:
1. Use a text-based `vertical-align` (`text-top`, `baseline`, etc…); and
2. Contain one or more block-level elements; and
3. Have collapsed borders; and
4. Those borders are 1px wide

An extra bottom border is rendered just below the bounding box of the text _in addition to_ the expected border where the rows meet. This does cause the text to shift upwards. This extra border is drawn only for the cells that DO NOT have the additional element inside. This behavior happens even when the added element is `position: fixed` or `position: absolute`. Additionally, the initial alignment box for the text is calculated incorrectly if a table initially uses `border-collapse: collapse` versus toggling that property separately via JS.

I've provided an example with a handful of small notes and examples of what's happening.

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