feat(layout-engine): balance columns at continuous section breaks (SD-2452)#2869
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feat(layout-engine): balance columns at continuous section breaks (SD-2452)#2869
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…-2452)
Implements ECMA-376 §17.18.77 column balancing for multi-column sections.
Word produces a minimum-height balanced layout at the end of a continuous
(and, empirically, next-page) multi-column section; SuperDoc was either
leaving content stacked in the first column or, in some layouts, producing
overlapping fragments.
The pagination pipeline now balances each multi-column section's last page
at layout time:
- layoutDocument builds a block -> section map by walking blocks in
document order and tracking the current section from the most recent
sectionBreak (pm-adapter only stamps attrs.sectionIndex on sectionBreak
blocks, not on content paragraphs).
- A new balanceSectionOnPage helper performs section-scoped balancing
with its own fragment-level positioning (no Y-grouping): fragments are
ordered by (x, y) in document order and each is treated as its own
block. The previous balancePageColumns grouped fragments by Y into
"rows," which collapsed fragments from different source columns at the
same Y and produced overlap.
- calculateBalancedColumnHeight is now a proper binary search for the
minimum column height H such that greedy left-to-right fill places
every block with every column <= H. This matches Word's left-heavy
packing preference (e.g. 7 blocks / 3 cols -> 3+3+1, not 2+2+3).
- A mid-page hook at forceMidPageRegion balances the ending section on
the current page before starting the new region, and collapses both
cursors to balanceResult.maxY so the next region begins just below the
balanced columns. Sections handled mid-page are tracked in
alreadyBalancedSections so the post-layout pass doesn't double-balance.
- The prior "last page of document" heuristic is replaced with a
per-section post-layout loop that balances each multi-column section's
last page, skipping sections already handled mid-page.
Tests:
- 11 new unit/integration tests covering the 5 SD-2452 fixtures
(2-col/3-col, equal and unequal heights, continuous and next-page
breaks, multi-page sections, explicit column-break opt-out).
- 614 layout-engine tests pass, 1737 pm-adapter tests pass,
11375 super-editor tests pass.
Visual validation against Microsoft Word for all 5 fixtures:
- Test 1 (6 paras / 2 cols): 3+3 exact match
- Test 2 (5 mixed / 2 cols): 2+3 exact match
- Test 3 (7 paras / 3 cols): 3+3+1 exact match
- Test 4 (13 paras / 2 cols): 7+6 exact match, overlap gone
- Test 5 (continuous + next-page): 3+2, 3+2 exact match
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…uction (SD-2452)
When a mid-page section break reduced the column count (e.g. 2-col ->
1-col for test 4's 13-paragraph fixture followed by OVERLAP CHECK), the
mid-page hook's forced-page-break guard ran before balancing:
if (columnIndexBefore >= newColumns.count) {
state = paginator.startNewPage();
}
// ... balance ran here, on the empty new page
At the section transition, columnIndexBefore=1 (paginator was in col 1)
and newColumns.count=1, so the guard forced a new page before balancing
had a chance to reposition the ending section's fragments. Balancing
then ran on the empty new page (no-op), the paginator placed the
post-columns single-column content on the new page, and the old page's
fragments were balanced by the post-layout pass. Net effect: columns
looked correct on page 0 but OVERLAP CHECK ended up on page 1, while
Word fits everything on one page.
The guard exists to prevent new 1-col content from overwriting earlier
column content on the same page. With balancing, that risk disappears:
all ending-section fragments are repositioned within the section's own
vertical region, and the cursor moves to maxY below the balanced
columns. The new region starts safely below.
Fix: balance first. Only fall through to the forced-page-break guard
when the ending section won't be balanced (single-col -> multi-col,
explicit column break, or no section-1 fragments on the page).
Test 4 now renders on a single page, matching Word:
- 7+6 balanced columns
- OVERLAP CHECK heading at y=758 (right below columns)
- "If this overlaps..." at y=794
- Total: 1 page (was 2)
All 5 SD-2452 fixtures now match Word's pagination exactly. 614
layout-engine tests still pass.
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Comparison Results PDF
SD-2452-page-by-page.pdf
Summary
Implements ECMA-376 §17.18.77 column balancing for multi-column sections. Word produces a minimum-height balanced layout at the end of a continuous (and empirically, next-page) multi-column section; SuperDoc was either leaving content stacked in the first column or, in some layouts, producing overlapping fragments.
Linear: SD-2452
What changed
layoutDocumentbuilds a block → section map by walking blocks in document order and tracking the current section from the most recentsectionBreak(pm-adapter only stampsattrs.sectionIndexon sectionBreak blocks, not on content paragraphs).balanceSectionOnPagehelper performs section-scoped balancing with its own fragment-level positioning (no Y-grouping). Fragments are ordered by(x, y)and each is treated as its own block. The previousbalancePageColumnsgrouped fragments by Y into rows, which collapsed fragments from different source columns at the same Y and produced overlap.calculateBalancedColumnHeightis a proper binary search for the minimumHsuch that greedy left-to-right fill places every block with every column ≤H. Matches Word's left-heavy packing preference (e.g. 7 blocks / 3 cols → 3+3+1, not 2+2+3).forceMidPageRegionbalances the ending section on the current page before starting the new region, and collapses both cursors tobalanceResult.maxYso the next region begins just below the balanced columns. Sections handled mid-page are tracked inalreadyBalancedSectionsso the post-layout pass doesn't double-balance.Results (Word vs SuperDoc)
Side-by-side PDF comparison available locally at
/tmp/sd-2452-fixtures/SD-2452-comparison.pdf(generated via newcompare-word-vs-superdocskill).Test plan
Demo tests
Fixtures
Plan is to upload these to the R2 corpus after the PR lands.