[Webkit-unassigned] [Bug 208845] New: Text layout miscalculates the number of lines that can fit in a single column

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=208845

            Bug ID: 208845
           Summary: Text layout miscalculates the number of lines that can
                    fit in a single column
           Product: WebKit
           Version: Safari 12
          Hardware: Macintosh
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: bya at 180g.co
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 393103

  --> https://bugs.webkit.org/attachment.cgi?id=393103&action=review

All (full) columns should have 10 lines

I have a multi-column layout where the column-height is evenly divided by the line-height. In the attached HTML, each column has a height 390px and contains text that has a line-height of 39px. I'd therefore expect exactly 10 lines in each (full) column.

That is true for the first column, but subsequent columns only display 9 lines. Somewhat surprisingly, this seems to be dependent upon the font-family and font-size. You can see this in the attached HTML. For Avenir, a font-size of 29px exhibits the bug; for Helvetica, a font size of 35px exhibits it.

This shouldn't be the case: since line-height is specified, changing these font attributes shouldn't affect the number of lines that can fit in a single column. In fact, InDesign, Chrome, and Edge all behave as expected.

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