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[css-break-3] Add the concept of grouping boxes #12805

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[css-break-3] Add the concept of grouping boxes #12805

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24 changes: 24 additions & 0 deletions css-break-3/Overview.bs
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Expand Up @@ -1003,6 +1003,30 @@ Splitting Boxes</h3>
</p>
</div>

<h4 id="grouping-boxes">
Grouping Boxes</h4>

<p>
Certain formatting contexts may establish what is known as a <dfn>grouping box</dfn>.
Other boxes can then be <dfn>assigned to</dfn> one or more of those [=grouping boxes=], which will encompass the entirety of the assigned box.
If a box is assigned to any [=grouping boxes=] and gets broken,
then for the purpose of filling any [=remaining fragmentainer extent=] as described above,
the [=remaining fragmentainer extent=] should be treated as if the [=fragmentainer=] it belongs to did not extend past the [=block-end=] of the [=grouping box=] that the break falls within.

<p>
Furthermore, if a box is assigned to any [=grouping boxes=] and the ancestors of those [=grouping boxes=] are broken between any two of the [=grouping boxes=], user agents should try to avoid creating box fragments for the [=assigned to|assigned box=], which span across that break.

<p class="note">
One way of accomplishing this is to make sure that within any [=assigned to|assigned box=] there is at least one break, preceding each point where there is a break between the [=grouping boxes=] that it is [=assigned to=].
Care should be taken to ensure that there is enough space below said break in the [=grouping box=] to fit any <a href="#valdef-box-decoration-break-clone">cloned margins/border/padding</a> so that they do not cause the [=assigned to|assigned box=] to extend beyond the [=grouping box=].

<p>
Grouping boxes are established in the following situations:
<ul>
<li>All table rows establish a grouping box. Any boxes for cells with a rowspan greater than 1 are [=assigned to=] the [=grouping boxes=] for all rows that they span.
<li>All grid rows in a [=grid container=] establish a [=grouping box=]. In this case, all [=grid items=] have their box [=assigned to=] the [=grouping boxes=] for all grid rows that their [=grid area=] covers.
</ul>

<h3 id="break-decoration">
Fragmented Borders and Backgrounds: the 'box-decoration-break' property</h3>

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