feat(layout-engine): balance columns at continuous section breaks (SD-2452)#2869
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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…cing-for-continuous-section
…D-2646) (#2930) * fix(pm-adapter): emit section break before non-paragraph nodes (SD-2646) Per ECMA-376 §17.6.17, a <w:sectPr> inside a paragraph defines the section that ENDS with that paragraph. All body children preceding it — paragraphs, tables, top-level drawings, SDTs — belong to that section. Section ranges were indexed purely by paragraph count, and section-break blocks were emitted only inside handleParagraphNode. A table that sat between two sectPr-marker paragraphs was emitted into the flow stream BEFORE the section break that declared its column config, so the layout engine laid it out under the prior section's settings. This is the root cause of IT-945 rendering a 114-row 2-col continuous table in column 0 across three pages with column 1 empty: the table was placed in the 1-col section, not the 2-col section. Fix: - Track nodeIndex over every top-level doc.content child in findParagraphsWithSectPr and SectionRange (alongside paragraphIndex, which SDT handlers still use for intra-SDT transitions). - Add maybeEmitNextSectionBreakForNode in sections/breaks.ts and call it from internal.ts's main dispatch loop BEFORE every top-level handler. Any non-paragraph node crossing a section boundary now triggers the break. - Section-model primer in pm-adapter/README.md with spec citations. Tests: 1739/1739 pass in pm-adapter (including new end-tagged.test.ts and integration test in index.test.ts asserting flow-block order). * fix(layout-engine): split dominant table at row boundary when balancing section-final page (SD-2646) The column balancer treats each fragment as an atomic block. A multi-page two-column continuous section's final page can end up with a single table fragment taller than totalSectionHeight / columnCount. The atomic-block binary search then places the whole table in one column and leaves the other empty — diverging from Word, which balances by splitting the table at a row boundary per ECMA-376 §17.18.77 ("a continuous section break balances the content of the previous section"). Fix: add splitDominantTableAtRowBoundary as a preprocessor inside balanceSectionOnPage. When the section has a single splittable table fragment larger than target, split it at the row whose cumulative height first meets or exceeds totalSectionHeight / columnCount. The two halves are inserted in place of the original; the rest of the balancer runs unchanged and naturally assigns one to each column. Also add getBalancingHeight so empty sectPr-marker paragraphs (measured lines with width=0) contribute 0 to balancing — matching Word's behavior of not rendering an empty line for such markers. This keeps both columns top-aligned on the section-final page. On IT-945: page 2 now splits 14/14 from y=96 in both columns, matching Word's top-alignment. Before this fix page 2 rendered all 28 remaining rows in col 1 with col 0 empty. Tests: strengthened existing "balances the section-ending page" test (it was passing trivially via `if (sectionFragments.length > 1)` guard). Added narrow-table multi-page regression test. 616/616 pass.
…cing-for-continuous-section
…cing-for-continuous-section
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@tupizz please double check layout testing and the below. I see some documents that have definitely regressed.
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@luccas-harbour can you pls work with Tadeu on getting this one to the finish line next week? Be extra careful around layout testing pls make sure no regressions! |
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hey @tupizz! nothing to add right now apart from Nick's comment. I'll have another look once those are addressed. also, note that the build is failing, which caused the behavior tests to fail running. thanks! |
4 participants
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.