[Webkit-unassigned] [Bug 48600] New: Multi-column layout unbalanced

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=48600

           Summary: Multi-column layout unbalanced
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: PC
        OS/Version: Mac OS X 10.5
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: steven at acko.net


WebKit's multi-column balancer appears to be buggy. I've attached two test-cases that show problematic behavior. Each test case shows a single multi-column container with one div inside. Clicking the button below adds additional divs to the container. As items are added, the columns are reflowed and various problems appear/disappear.

Note: due to apparent lack of support for "-webkit-column-break-inside: avoid" in WebKit, the test-cases look weird when items < columns. For the purposes of this bug report, these scenarios may be ignored.

When the number of items is not divisible by the number of columns, the columns are not balanced from left-to-right as one would expect. Instead, the excess items are either added to the last column or 'dithered' between columns. There also appear to be related off-by-one errors where a column is wrapped one pixel too soon or late, which might be the reason for the unbalanced layout.

Column-test-1: Each item is a simple div with text.
Result:
- 2 columns: off-by-one error
- 3 columns: unbalanced last column, off-by-one error
- 4 columns: dithering
- 5 columns: dithering
- 6 columns: unbalanced last two columns, dithering, off-by-one errors
- 7 columns: unbalanced last two columns, dithering, off-by-one error
- 8 columns: unbalanced last column, dithering, off-by-one errors

Column-test-2: Each item contains two elements, one of which floats.
Result:
- 2 columns: last column unbalanced
- 3 columns: last column twice unbalanced
- 4 columns: last 2 columns unbalanced
- 5 columns: last 3 columns unbalanced
- 6 columns: last 3 columns unbalanced
- 7 columns: last 4 columns unbalanced
- 8 columns: last 5 columns unbalanced

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