fix: keep-with-next reserves the lines widow control would keep

A heading only moved down when its successor's *first* line no longer fit. But
widow-orphan control carries a lone first line over anyway, so the heading was
left stranded at the page foot — LibreOffice moves the pair. The fit test now
asks for MIN_KEPT_LINES of the successor.

docs/architecture/pagination.md

@@ -28,7 +28,7 @@ ping-pong and keeps the current layout; only an edit or a forced recalc forgets
 rewrites its page numbers on every `pm-pagecount`, so resetting on any document change would
 leave nothing to catch).
 
-**Keeping blocks together** (`pageBreak.ts` attrs, both read from the direct property *and* the paragraph style): `keepNext` (`w:keepNext`, `fo:keep-with-next`) moves a block down when its successor's first line no longer fits below it — headings do that in Word and LibreOffice anyway, so the attr only marks the others. `keepLines` (`w:keepLines`, `fo:keep-together`) makes an otherwise splittable block paginate atomically, unless it is taller than the page slot, where Word splits it too.
+**Keeping blocks together** (`pageBreak.ts` attrs, both read from the direct property *and* the paragraph style): `keepNext` (`w:keepNext`, `fo:keep-with-next`) moves a block down when the successor's first `MIN_KEPT_LINES` no longer fit below it — one line is not enough, because widow-orphan control would carry the second one over and strand the block anyway (measured on the Math Guide: a heading left alone at the foot of page 8) — headings do that in Word and LibreOffice anyway, so the attr only marks the others. `keepLines` (`w:keepLines`, `fo:keep-together`) makes an otherwise splittable block paginate atomically, unless it is taller than the page slot, where Word splits it too.
 
 **Line breaking:** `editor.css` overrides prosemirror-view's `white-space: break-spaces` back to `pre-wrap` on `.paper .tiptap`. Under `break-spaces` a line-end space may not hang into the margin and counts toward the line width, so lines break one word earlier than in Word/LibreOffice whenever a line reaches within a space width of the margin.
 

src/lib/editor/extensions/pageBreaks.ts

@@ -505,11 +505,14 @@ export const PageBreaks = Extension.create({
 
         // `minLines` is the widow-orphan minimum, or 1 for a leaf taller than one page
         // slot — there the rule is unsatisfiable and splitting beats overflowing.
-        // What a keep-with-next block has to fit below itself: the successor's first
-        // line, in unscaled doc px. An empty block falls back to its whole height.
-        function firstLineHeight(el: HTMLElement, scale: number): number {
-          const first = getLineRects(el)[0];
-          return first ? (first.bottom - first.top) / scale : el.offsetHeight;
+        // What a keep-with-next block has to fit below itself, in unscaled doc px: as
+        // many lines of the successor as widow-orphan control would keep together
+        // anyway. An empty block falls back to its whole height.
+        function firstLinesHeight(el: HTMLElement, scale: number, want: number): number {
+          const lines = getLineRects(el);
+          if (!lines.length) return el.offsetHeight;
+          const last = lines[Math.min(want, lines.length) - 1];
+          return (last.bottom - lines[0].top) / scale;
         }
 
         function findLineSplit(
@@ -1152,9 +1155,10 @@ export const PageBreaks = Extension.create({
               // What the pair needs below this block: its own space below, then the
               // successor's space above (its padding; an atomic leaf's height has it
               // already) and its first line.
+              const wantLines = next.el.getAttribute('data-widow-control') === 'false' ? 1 : MIN_KEPT_LINES;
               const needed = next.kind === 'atomic'
                 ? Math.min(next.naturalHeight, CONTENT_HEIGHT)
-                : firstLineHeight(next.el, scale) + (parseFloat(getComputedStyle(next.el).paddingTop) || 0);
+                : firstLinesHeight(next.el, scale, wantLines) + (parseFloat(getComputedStyle(next.el).paddingTop) || 0);
               if (effectiveBottom + (leaf.spaceAfter ?? 0) + needed > contentEnd) {
                 const target = pageContentStart(page + 1, vm.top, CYCLE_PX);
                 const { docPos, row } = leafSpacer(leaf);