Share AutoFit donor reductions equally (#2464)

Co-authored-by: Brandon Thomas <echelon@gmail.com>

crates/layout/src/table.rs

@@ -1217,9 +1217,49 @@ fn autofit(grid: &mut [f32], content: &[(f32, f32)], t: &Table, avail: f32) {
     if short < 0.01 {
         return;
     }
-    let spare: f32 = grid.iter().zip(content).map(|(w, c)| (*w - c.0).max(0.0)).sum();
-    let give = (short / spare.max(0.01)).min(1.0);
+    // Donors give equal widths until they reach their own content minimum.
+    let mut rooms: Vec<_> = grid.iter().zip(content).map(|(w, c)| *w - c.0).filter(|room| *room > 0.0).collect();
+    rooms.sort_by(f32::total_cmp);
+    let mut remaining = short;
+    let mut share = 0.0;
+    for (i, room) in rooms.iter().enumerate() {
+        share = remaining / (rooms.len() - i) as f32;
+        if share <= *room {
+            break;
+        }
+        remaining -= room;
+    }
     for (w, c) in grid.iter_mut().zip(content) {
-        *w = if *w < c.0 { c.0 } else { *w - (*w - c.0) * give };
+        *w = if *w < c.0 { c.0 } else { *w - (*w - c.0).min(share) };
+    }
+}
+
+#[cfg(test)]
+mod width_tests {
+    use super::*;
+
+    #[test]
+    fn autofit_shares_donor_loss() {
+        let table = Table::new(1, 4, 260.0);
+        let mut grid = [30.0, 120.0, 20.0, 90.0];
+        let content = [(28.0, 28.0), (40.0, 120.0), (40.0, 40.0), (30.0, 90.0)];
+        autofit(&mut grid, &content, &table, 260.0);
+        assert_eq!(grid, [28.0, 111.0, 40.0, 81.0]);
+    }
+
+    #[test]
+    fn autofit_exhausts_donors() {
+        let table = Table::new(1, 2, 50.0);
+        let mut grid = [30.0, 20.0];
+        autofit(&mut grid, &[(25.0, 25.0), (40.0, 40.0)], &table, 50.0);
+        assert_eq!(grid, [25.0, 40.0]);
+    }
+
+    #[test]
+    fn autofit_keeps_fitting_widths() {
+        let table = Table::new(1, 3, 120.0);
+        let mut grid = [30.0, 40.0, 50.0];
+        autofit(&mut grid, &[(10.0, 10.0), (20.0, 20.0), (30.0, 30.0)], &table, 120.0);
+        assert_eq!(grid, [30.0, 40.0, 50.0]);
     }
 }

docs/layout-parity.md

@@ -1,6 +1,6 @@
 # Layout and pagination parity with Microsoft Word
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-17 · **Change:** trivial (automatic space between Asian text and Latin text or digits, #2358; previously trivial: hint-reliant CJK outlines; previously trivial: positional tabs go to the margins or indents, #2367; previously trivial: field results with right-to-left text read right to left, #1670; previously trivial: a list bullet no installed font can draw shows the level's default bullet, #1683; previously trivial: a missing Thai font is drawn in one Thai face for the whole run, #1549; previously trivial: the caret after spaces hanging past the margin stays at the margin, #2146; previously trivial: covered black-circle fallback before UI sans; previously trivial: resolved split-cell continuation top insets; previously trivial: Symbol PCL6 alias uses Symbol encoding; previously trivial: exact full font names select their installed face; previously trivial: Arial Unicode MS uses proportional fallbacks; previously trivial: DrawingML outline dashes, including zero-width paths; previously trivial: consistent cell widths override a stale table grid; previously trivial: multi-paragraph field instructions no longer add blank lines; previously trivial: page-break-only list paragraphs hide their labels; previously trivial: floating-table overlap ignores empty line-break lines; previously trivial: paragraph spacing adds up when the document turns HTML auto spacing off; previously trivial: an inherited footer sizes its percentage tables by its own section; previously trivial: an empty section mark stays on a full page; previously trivial: a fitting row with less room than its minimum height moves on whole; oversized rows still split; previously trivial: text past an exact-height row is cut off; previously trivial: a list number stays with text after a leading page break; previously trivial: a bookmark after a page break takes no line; previously trivial: a page break followed by a page break before or a next-page section starts one page; previously trivial: All Caps and Small Caps draw ß as SS, #1581; previously trivial: justified vertical page alignment, #1320; previously trivial: page numbers restart at 0; previously trivial: a list label moves with its first line when the paragraph is centred or end-aligned (#1159); previously trivial: PAGE fields in a paragraph split across pages show the page they are on, #1352; previously trivial: a dragged floating object anchors in the column it lands in, #884; previously trivial: a paragraph ending on a page with footnotes keeps its space after above the separator, #1116; previously trivial: symbol-font text drawn as Unicode symbols, #776; previously trivial: large-table scrolling measured and fixed, #811; previously trivial: text boxes use their own margins; Resize shape to fit text never cuts text, #743; previously trivial: complex-script shaping and Myanmar line breaking, #604; previously trivial: body page totals, table-cell footnotes and bookmark link pages fixed, #450 #451 #455; previously major: first version; line breaking, pagination and object placement checked against Word's behaviour from the layout source on origin/main) · **Target:** Microsoft Word (Microsoft 365) for Mac 16.113.4
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-17 · **Change:** trivial (AutoFit donors lose equal widths down to their content minima; previously trivial: automatic space between Asian text and Latin text or digits, #2358; previously trivial: hint-reliant CJK outlines; previously trivial: positional tabs go to the margins or indents, #2367; previously trivial: field results with right-to-left text read right to left, #1670; previously trivial: a list bullet no installed font can draw shows the level's default bullet, #1683; previously trivial: a missing Thai font is drawn in one Thai face for the whole run, #1549; previously trivial: the caret after spaces hanging past the margin stays at the margin, #2146; previously trivial: covered black-circle fallback before UI sans; previously trivial: resolved split-cell continuation top insets; previously trivial: Symbol PCL6 alias uses Symbol encoding; previously trivial: exact full font names select their installed face; previously trivial: Arial Unicode MS uses proportional fallbacks; previously trivial: DrawingML outline dashes, including zero-width paths; previously trivial: consistent cell widths override a stale table grid; previously trivial: multi-paragraph field instructions no longer add blank lines; previously trivial: page-break-only list paragraphs hide their labels; previously trivial: floating-table overlap ignores empty line-break lines; previously trivial: paragraph spacing adds up when the document turns HTML auto spacing off; previously trivial: an inherited footer sizes its percentage tables by its own section; previously trivial: an empty section mark stays on a full page; previously trivial: a fitting row with less room than its minimum height moves on whole; oversized rows still split; previously trivial: text past an exact-height row is cut off; previously trivial: a list number stays with text after a leading page break; previously trivial: a bookmark after a page break takes no line; previously trivial: a page break followed by a page break before or a next-page section starts one page; previously trivial: All Caps and Small Caps draw ß as SS, #1581; previously trivial: justified vertical page alignment, #1320; previously trivial: page numbers restart at 0; previously trivial: a list label moves with its first line when the paragraph is centred or end-aligned (#1159); previously trivial: PAGE fields in a paragraph split across pages show the page they are on, #1352; previously trivial: a dragged floating object anchors in the column it lands in, #884; previously trivial: a paragraph ending on a page with footnotes keeps its space after above the separator, #1116; previously trivial: symbol-font text drawn as Unicode symbols, #776; previously trivial: large-table scrolling measured and fixed, #811; previously trivial: text boxes use their own margins; Resize shape to fit text never cuts text, #743; previously trivial: complex-script shaping and Myanmar line breaking, #604; previously trivial: body page totals, table-cell footnotes and bookmark link pages fixed, #450 #451 #455; previously major: first version; line breaking, pagination and object placement checked against Word's behaviour from the layout source on origin/main) · **Target:** Microsoft Word (Microsoft 365) for Mac 16.113.4
 
 A document only "opens correctly" if its pages break where Word breaks them: the same lines, the
 same page count, footnotes and pictures on the same pages. This checklist covers line breaking,
@@ -180,6 +180,7 @@ and save, in raster output and in PDF. Synthetic controls cover red rectangles,
 connectors. Custom lists are capped at 64 pairs; painted spans are bounded to 0.01–20000 pt.
 Theme-inherited dash styles, line caps, ink and open freeform strokes are not covered by this change.
 
+When content exceeds a preferred column width, AutoFit donors lose equal widths until they reach their content minima. Original synthetic Word probes cover donor reductions; cached field refresh and wrapping remain separate gaps.
 ## Revision history
 
 | Date | Change | Summary |
@@ -196,6 +197,7 @@ Theme-inherited dash styles, line caps, ink and open freeform strokes are not co
 | 2026-10-16 | trivial | Positional tabs (`w:ptab`) are laid out at the margins or indents they name, not at the next tab stop (#2367) |
 | 2026-10-16 | trivial | Hint-reliant CJK fonts execute TrueType contour assembly; Windows DFKai-SB no longer loses strokes. Synthetic local-font regression; no font binaries bundled. |
 | 2026-10-16 | trivial | `w:autoSpaceDE` and `w:autoSpaceDN` are laid out: a quarter em between Asian text and Latin text or digits (#2358) |
+| 2026-10-16 | trivial | AutoFit donors lose equal widths, bounded by content minima |
 | 2026-10-14 | trivial | Row table border exceptions draw without leaking into other rows |
 | 2026-10-14 | trivial | Consistent cell widths override a stale table grid |
 | 2026-10-14 | trivial | Paragraphs inside complex-field instructions take no flow height; empty result paragraphs keep their line. |