diff --git a/apps/web/src/components/ComposerPromptEditorTiptap.tsx b/apps/web/src/components/ComposerPromptEditorTiptap.tsx
index 1986cc888efba..daaad4c3b6892 100644
--- a/apps/web/src/components/ComposerPromptEditorTiptap.tsx
+++ b/apps/web/src/components/ComposerPromptEditorTiptap.tsx
@@ -89,6 +89,7 @@ import {
   markdownToFlat,
   pmToFlat,
   serializeEditorDoc,
+  serializedMarkdownOffset,
   serializeSelection,
   skillChipReplacements,
   splitOrLiftListItem,
@@ -1593,7 +1594,7 @@ function ComposerPromptEditorTiptapInner(
         },
       );
     }
-    selectMarkdownOffset(editor, normalizedExpandedCursor);
+    selectMarkdownOffset(editor, value, normalizedExpandedCursor);
     const map = serializeEditorDoc(editor.state.doc);
     if (isFocused) scrollTiptapCaretIntoView(editor);
     if (pendingCitation) {
@@ -1666,7 +1667,7 @@ function ComposerPromptEditorTiptapInner(
     const boundedCursor = clampEditorCursor(snapshotRef.current.value, nextCursor);
     const expandedCursor =
       exactExpandedCursor ?? expandEditorCursor(snapshotRef.current.value, boundedCursor);
-    selectMarkdownOffset(editor, expandedCursor);
+    selectMarkdownOffset(editor, snapshotRef.current.value, expandedCursor);
     scrollTiptapCaretIntoView(editor);
     if (boundedCursor === snapshotRef.current.cursor) return;
     snapshotRef.current = {
@@ -1892,10 +1893,13 @@ function insertMarkdownParagraphs(
 }
 
 /** Placing the caret can show Markdown markers, so place it again in the shown source. */
-function selectMarkdownOffset(editor: TiptapEditor, offset: number): void {
+function selectMarkdownOffset(editor: TiptapEditor, source: string, offset: number): void {
   for (let pass = 0; pass < 2; pass += 1) {
     const map = serializeEditorDoc(editor.state.doc);
-    const pos = flatToPm(map, markdownToFlat(map, offset));
+    const pos = flatToPm(
+      map,
+      markdownToFlat(map, serializedMarkdownOffset(source, map.value, offset)),
+    );
     const { selection } = editor.state;
     if (pass > 0 && selection.empty && selection.from === pos) return;
     editor.commands.setTextSelection(pos);
diff --git a/apps/web/src/composer-rich-text-doc.test.ts b/apps/web/src/composer-rich-text-doc.test.ts
index fcdb8165662b5..df78eac50b152 100644
--- a/apps/web/src/composer-rich-text-doc.test.ts
+++ b/apps/web/src/composer-rich-text-doc.test.ts
@@ -30,6 +30,7 @@ import {
   markdownToFlat,
   pmToFlat,
   serializeEditorDoc,
+  serializedMarkdownOffset,
   serializeSelection,
   skillChipReplacements,
   stepCaretAcrossStyledEdge,
@@ -597,6 +598,53 @@ describe("composer rich text document model", () => {
     });
   });
 
+  describe("a dash rule after a paragraph", () => {
+    const paragraph = (text: string) => ({ type: "paragraph", content: [{ type: "text", text }] });
+    const rule = (source: string) => ({ type: "horizontalRule", attrs: { source } });
+    const serialize = (...content: object[]) => {
+      const doc = ProseMirrorNode.fromJSON(schema, { type: "doc", content });
+      doc.check();
+      return { doc, map: serializeEditorDoc(doc) };
+    };
+
+    it("is written after a blank line, or Markdown reads the paragraph as a heading", () => {
+      const settled = "text\n\n---\nmore";
+      expect(roundTrip("text\n---\nmore").value).toBe(settled);
+      expect(roundTrip(settled).value).toBe(settled);
+    });
+
+    it("keeps every offset mapped through the blank line", () => {
+      const { doc, map } = serialize(paragraph("text"), rule("---"), paragraph("more"));
+      expect(map.value).toBe("text\n\n---\nmore");
+      for (let flat = 0; flat <= map.docLength; flat += 1) {
+        expect(markdownToFlat(map, flatToMarkdown(map, flat))).toBe(flat);
+      }
+      // The blank line is not a place to type: it clamps to the rule.
+      expect(flatToMarkdown(map, "text".length)).toBe("text".length);
+      expect(markdownToFlat(map, "text\n".length)).toBe("text".length + 1);
+      expect(flatToMarkdown(map, "text".length + 1)).toBe("text\n\n---".length);
+      expect(flatToPm(map, "text".length + 1)).toBe(doc.child(0).nodeSize + doc.child(1).nodeSize);
+    });
+  });
+
+  it.each([
+    ["text\n---\nmore", "text\n---\nm".length, "text\n\n---\nm".length],
+    ["before\n> quoted\nafter", "before\n> quoted\na".length, "before\n> quoted\n\na".length],
+    ["__x__\n---\nmore", "__x__\n---\nm".length, "**x**\n\n---\nm".length],
+    [
+      "*a*_b_\n> q\nx\n> q\nabcde",
+      "*a*_b_\n> q\nx\n> q\na".length,
+      "*a*_b_\n> q\nx\n> q\na".length,
+    ],
+    ["a\n---\n", "a".length, "a".length],
+  ])("restores a caret in the glued draft %j on the same character", (value, caret, settled) => {
+    const map = roundTrip(value);
+    const restore = (offset: number) =>
+      markdownToFlat(map, serializedMarkdownOffset(value, map.value, offset));
+    expect(restore(value.length)).toBe(map.docLength);
+    expect(restore(caret)).toBe(markdownToFlat(map, settled));
+  });
+
   it.each([
     "---",
     "***",
@@ -605,7 +653,8 @@ describe("composer rich text document model", () => {
     "* * *",
     "-----",
     "---   ",
-    "text\n---\nmore",
+    "text\n***\nmore",
+    "text\n- - -\nmore",
     "- item\n---\n- item two",
     "```\n---\n```",
     "--",
@@ -651,7 +700,7 @@ describe("composer rich text document model", () => {
 
   it.each([
     ["---", "---"],
-    ["a\n---", "a\n---"],
+    ["a\n---", "a\n\n---"],
     ["---\n", "---"],
   ])("gives the caret a line after the final rule of %j, written %j", (value, expected) => {
     const map = roundTrip(value);
diff --git a/apps/web/src/composer-rich-text-doc.ts b/apps/web/src/composer-rich-text-doc.ts
index 417397e88ccc5..94acc41bf0eea 100644
--- a/apps/web/src/composer-rich-text-doc.ts
+++ b/apps/web/src/composer-rich-text-doc.ts
@@ -1051,14 +1051,26 @@ function pushBreakRun(acc: RichAccumulator, position?: number): void {
   acc.md += 1;
 }
 
-// Separate prose from quotes to avoid lazy continuation; an empty paragraph already separates them.
-function needsBlankLineAfterQuote(previous: ProseMirrorNode, block: ProseMirrorNode): boolean {
+function horizontalRuleSource(block: ProseMirrorNode): string {
+  const source = (block.attrs as Record<string, unknown>).source;
+  return typeof source === "string" && source ? source : "---";
+}
+
+// A blank line keeps prose out of a quote (lazy continuation) and a dash rule from
+// underlining the line above into a setext heading; an empty paragraph already separates them.
+function needsBlankLineBetween(previous: ProseMirrorNode, block: ProseMirrorNode): boolean {
+  if (previous.type.name === "blockquote") {
+    return block.type.name === "paragraph" && block.content.size > 0;
+  }
   return (
-    previous.type.name === "blockquote" && block.type.name === "paragraph" && block.content.size > 0
+    block.type.name === "horizontalRule" &&
+    previous.type.name === "paragraph" &&
+    previous.content.size > 0 &&
+    /^-+[ \t]*\r?$/.test(horizontalRuleSource(block))
   );
 }
 
-// Blank-line runs own no document characters; offsets clamp to the paragraph start.
+// Blank-line runs own no document characters; offsets clamp to the start of the next block.
 function pushBlankLineRun(acc: RichAccumulator, pmPos: number): void {
   acc.runs.push({
     kind: "prefix",
@@ -1074,6 +1086,27 @@ function pushBlankLineRun(acc: RichAccumulator, pmPos: number): void {
   acc.md += 1;
 }
 
+// Moves an offset into a parsed `value` past the blank lines its serialization inserted,
+// treating other differences (`__` written as `**`) as same-length rewrites. When the two
+// strings do not align that way end to end, the offset is returned unmapped.
+export function serializedMarkdownOffset(
+  value: string,
+  serialized: string,
+  offset: number,
+): number {
+  if (value === serialized) return offset;
+  let index = 0;
+  let mapped = 0;
+  let target = -1;
+  while (index < value.length && mapped < serialized.length) {
+    if (index === offset && target < 0) target = mapped;
+    if (value[index] === serialized[mapped] || serialized[mapped] !== "\n") index += 1;
+    mapped += 1;
+  }
+  if (index !== value.length || mapped !== serialized.length) return offset;
+  return target < 0 ? mapped + (offset - index) : target;
+}
+
 function appendInlineRuns(
   container: ProseMirrorNode,
   contentStart: number,
@@ -1502,7 +1535,7 @@ function serializeDoc(doc: ProseMirrorNode): RichDocMap {
   let pmBlockStart = 0;
   blocks.forEach((block, blockIndex) => {
     if (blockIndex > 0 && !isTrailingLineAfterRule(doc, blockIndex)) pushBreakRun(acc);
-    if (blockIndex > 0 && needsBlankLineAfterQuote(blocks[blockIndex - 1]!, block)) {
+    if (blockIndex > 0 && needsBlankLineBetween(blocks[blockIndex - 1]!, block)) {
       pushBlankLineRun(acc, pmBlockStart + 1);
     }
     if (LIST_NODE_NAMES.has(block.type.name)) {
@@ -1530,8 +1563,7 @@ function serializeDoc(doc: ProseMirrorNode): RichDocMap {
       acc.md += prefix.length;
       appendInlineRuns(block, pmBlockStart + 1, acc);
     } else if (block.type.name === "horizontalRule") {
-      const attrs = block.attrs as Record<string, unknown>;
-      const source = typeof attrs.source === "string" && attrs.source ? attrs.source : "---";
+      const source = horizontalRuleSource(block);
       acc.runs.push({
         kind: "prefix",
         flatStart: acc.flat,