Bug 287279

Minimum reproducible example: ``` <div style="column-count: 2"> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div style="break-before: column"></div> </div> ``` Expected behavior: All ten divs should appear in the first column, second column should contain the <div style="break-before: column"> Actual behavior: The 10 divs get split into left/right columns, 5 on each side. # Other examples ``` <div style="column-count: 2"> <div>some text</div> <div>some text</div> <div>some text</div> <div style="break-before: column"></div> <div></div> </div> ``` Expected: 3 "some text"s in first column, empty div in second column. Actual: 2 "some text" in first column, 1 "some text", and the two empty divs in second column. ``` <div id="app" style="column-count: 3"> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div style="break-before: column"></div> <div>some text</div> <div style="break-before: column"></div> </div> ``` Expected: 5 "some text" in first column, 1 "some text" in second column, <div style="break-before: column"> in the 3rd column. Actual: 3 in first column, 2 in second column, 1 in third column. ``` <div id="app" style="column-count: 3"> <div style="break-after: column"></div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div>some text</div> <div style="break-before: column"></div> </div> ``` Expected: Empty first column, 6 "some text" in 2nd column, <div style="break-before: column"> in the 3rd column Actual: 3 in first column, 3 in second column, i assume <div style="break-before: column"> in the third column, but Safari DevTools make this difficult to analyze since it doesn't seem to show the placement of zero-area elements.