fix: floor a DOCX anchor's paragraph posOffset at one twip

Anchored exactly on the paragraph top (posOffset 0), LibreOffice's wrap
layout derails and paints a neighbouring inline text box's content
outside its shape; LO itself never writes less than a twip there.
Probed by bisecting a broken box against LO's own DOCX export of the
same document. The importer rounds the sub-visible offset back to none,
so nothing accretes on the round trip.

docs/architecture/frames.md

@@ -60,6 +60,9 @@ double arrow.
   an undeclared `draw:` prefix there makes the whole file unreadable.
 - **DOCX** keeps the frame and flips it (`<a:xfrm flipV="1">`), the heads riding the
   shape's `<a:ln>` as `a:headEnd`/`a:tailEnd`, and writes no `wps:txbx` at all.
+- An anchored frame's DOCX `positionV` posOffset is floored at one twip: on exactly 0,
+  LibreOffice's wrap layout paints a neighbouring inline box's text outside its shape
+  (probed; LO never writes less itself). The importer reads the twip back as no offset.
 ## Freeform outlines (`shapePath`)
 
 A drawing this editor offers no tool to author — a polygon, a polyline, a bezier curve,

src/lib/export/docx.ts

@@ -739,6 +739,14 @@ function decodeDataUri(src: string): { bytes: Uint8Array; type: 'png' | 'jpg' |
   }
 }
 
+// A frame sitting exactly on its paragraph's top edge derails LibreOffice's wrap
+// layout around a neighbouring inline box (text paints outside the box, probed);
+// LO itself never writes a paragraph offset below one twip.
+function paraOffsetEmu(cm: number): number {
+  const emu = Math.round(cm * 360000);
+  return emu >= 0 && emu < 635 ? 635 : emu;
+}
+
 // offsetCm places the frame in the text column (Word's posOffset); without one it is
 // flush to its side. offsetYCm is how far below the anchor paragraph it sits.
 function floatingFor(wrap: string, offsetCm: number | null, offsetYCm: number | null, alignH?: string | null, distCm?: number | null, inFront?: boolean): IFloating | undefined {
@@ -747,7 +755,7 @@ function floatingFor(wrap: string, offsetCm: number | null, offsetYCm: number |
   const margins = distCm ? { left: Math.round(distCm * 360000), right: Math.round(distCm * 360000) } : undefined;
   const verticalPosition = {
     relative: VerticalPositionRelativeFrom.PARAGRAPH,
-    offset: offsetYCm != null ? Math.round(offsetYCm * 360000) : 0,
+    offset: paraOffsetEmu(offsetYCm ?? 0),
   };
   if (wrap === 'through') {
     // Word's in-front-of / behind-text: no wrap at all, and behindDoc names which side
@@ -1243,7 +1251,7 @@ function textBoxDrawingXml(box: TextBoxDocx, index: number, parts: TxbxParts): s
     ` simplePos="0" relativeHeight="${251658240 + index}" behindDoc="${box.wrap === 'through' ? 1 : 0}" locked="0" layoutInCell="1" allowOverlap="${box.wrap === 'through' ? 1 : 0}">` +
     `<wp:simplePos x="0" y="0"/>` +
     `<wp:positionH relativeFrom="margin">${posH}</wp:positionH>` +
-    `<wp:positionV relativeFrom="paragraph"><wp:posOffset>${emu(box.offsetYCm ?? 0)}</wp:posOffset></wp:positionV>` +
+    `<wp:positionV relativeFrom="paragraph"><wp:posOffset>${paraOffsetEmu(box.offsetYCm ?? 0)}</wp:posOffset></wp:positionV>` +
     `${extent}${wrapEl}${docPr}${graphic}</wp:anchor></w:drawing>`
   );
 }

src/lib/import/docx.ts

@@ -1994,7 +1994,11 @@ function anchorOffsetY(anchor: Element): number | null {
   const from = posV?.getAttribute('relativeFrom');
   if (from !== 'paragraph' && from !== 'line') return null;
   const off = parseInt(posV?.getElementsByTagNameNS(WP, 'posOffset')[0]?.textContent ?? '', 10);
-  return Number.isFinite(off) && off > 0 ? round2(off / 360000) : null;
+  if (!Number.isFinite(off)) return null;
+  // The exporter floors this offset at one twip (LO derails on 0); sub-visible
+  // remainders round back to none, not to a 0 that would accrete as an attribute.
+  const cm = round2(off / 360000);
+  return cm > 0 ? cm : null;
 }
 
 // The frame's own x in the text column, cm from its left edge. null where the file

tests/lo-render-consistency.test.ts

@@ -52,9 +52,9 @@ describe.skipIf(!has('soffice') || !has('pdftoppm'))('ODT and DOCX render alike
     const docxPages = renderPages('doc.docx', 'docx');
     expect(docxPages.length, 'both formats paginate to the same page count').toBe(odtPages.length);
 
-    // Ratchet thresholds over the measured status quo (7.6/10.8/4.4/0.3/0.2% on
-    // 2026-08-28; the page-2 cluster is floating frames + the styled table's row
-    // split) — tighten them as the cross-format deviations get fixed.
+    // Ratchet thresholds over the measured status quo (7.6/11.5/4.1/0.3/0.2% on
+    // 2026-08-28). The page-2 rest is LibreOffice-only: it paints a custom-shape's
+    // text at the line's pre-wrap position, and it drops OMML formulas headless.
     const perPage: number[] = [];
     for (let i = 0; i < odtPages.length; i++) {
       const a = pgm(odtPages[i]);