[csswg-drafts] [css-break-3] Should broken boxes always be extended to fill the gap? (#12801)

Psychpsyo has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-break-3] Should broken boxes always be extended to fill the gap? ==
[CSS Break 3](https://www.w3.org/TR/css-break-3/) says, in [5.3. Splitting Boxes](https://www.w3.org/TR/css-break-3/#box-splitting), that when a box gets split, it's block size is effectively increased. This leads to its backgrounds and borders being painted into the remaining empty space in that fragmentainer.

This usually seems desirable, but it gets ugly when a table cell with the `rowspan` attribute gets broken due to the rows that it spans falling into different fragmentainers. In this case, the empty space that comes to exist between those rows has nothing in it, apart from the `rowspan`-cell, which I'd expect to only render next to the rows that it spans.

I think there should either be an exception for these cells, or a CSS property that can control this behavior, similar to `box-decoration-break`. Alternatively, `box-decoration-break` could be extended to accept something like `[ slice | clone ] || skip-gap` instead of just `slice | clone`. This new behavior should, in my opinion, be applied to rowspan cells by default, so this could be added to the UA stylesheet:
```css
td[rowspan], th[rowspan] {
    box-decoration-break: skip-gap;
}
```
The UA stylesheet addition would be a breaking change, though currently only Safari manages to properly break these cells at all. Both Chrome and Firefox fail to lay them out correctly in different ways. (Chrome doesn't extend the height on the rowspanned cell so it doesn't reach the end of the second row and Firefox draws the background only inside the first fragmentainer and only next to the first row.)

TL;DR:
<img width="1310" height="653" alt="Image" src="https://github.com/user-attachments/assets/789bbcb1-69ab-4781-872d-0378335b3d6f" />

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/12801 using your GitHub account


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Received on Monday, 15 September 2025 10:47:09 UTC