[Webkit-unassigned] [Bug 242313] New: margin-bottom wraps into next column if columns are of uneven height

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=242313

            Bug ID: 242313
           Summary: margin-bottom wraps into next column if columns are of
                    uneven height
           Product: WebKit
           Version: Safari 15
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kimberly.grey at digital.cabinet-office.gov.uk
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 460652

  --> https://bugs.webkit.org/attachment.cgi?id=460652&action=review

HTML example (duplicate of Codepen)

## Steps to reproduce

1. Open the Codepen at https://codepen.io/querkmachine/pen/ExEVVBL or the attached HTML file in Safari. 
2. Ensure the viewport is wide enough for the content to be presented in two columns without text wrapping.

## Expected behaviour

List items 1 to 7 appear in the first column, list items 8 to 13 appear in the second column. 

The tops of both columns are vertically aligned with one another. 

## Actual behaviour 

List items 1 to 7 appear in the first column, list items 8 to 13 appear in the second column.

The top of the second column appears 20 pixels below the top of the first column, as the `margin-bottom` from list item 7 is being rendered above list item 8. 

In this example, the issue is triggered by a combination of an odd number of list items and the last list item having the bottom margin reset to 0, creating a large enough difference in column lengths for the margin to wrap.

I could also reproduce this in the Safari 16 beta. The columns appear as expected in Firefox and Chromium browsers. 

## Potentially related bugs

- https://bugs.webkit.org/show_bug.cgi?id=17887
- https://bugs.webkit.org/show_bug.cgi?id=105411 
- https://bugs.webkit.org/show_bug.cgi?id=212505

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