diff --git a/desktop/src/features/forum/ui/ForumComposer.tsx b/desktop/src/features/forum/ui/ForumComposer.tsx
index 012799b37..94432712f 100644
--- a/desktop/src/features/forum/ui/ForumComposer.tsx
+++ b/desktop/src/features/forum/ui/ForumComposer.tsx
@@ -96,40 +96,47 @@ export function ForumComposer({
       setContent(markdown);
       contentRef.current = markdown;
 
-      const { cursor } = richText.getTextAndCursor();
+      const { cursor } = richText.getPlainTextAndCursor();
       mentions.updateMentionQuery(text, cursor);
       channelLinks.updateChannelQuery(text, cursor);
     },
   });
 
   // ── Mention / channel autocomplete insertion ────────────────────────
+  // Native ProseMirror transactions — no markdown round-trip.
   const applyMentionInsert = React.useCallback(
     (suggestion: MentionSuggestion) => {
-      const { text, cursor } = richText.getTextAndCursor();
-      const result = mentions.insertMention(suggestion, text, cursor);
-      richText.setContentWithTrailingSpace(result.nextContent);
-      setContent(result.nextContent);
-      contentRef.current = result.nextContent;
+      const { cursor } = richText.getPlainTextAndCursor();
+      const { replaceFromOffset, replaceToOffset, insertText } =
+        mentions.insertMention(suggestion, cursor);
+      richText.replacePlainTextRange(
+        replaceFromOffset,
+        replaceToOffset,
+        insertText,
+      );
     },
     [
       mentions.insertMention,
-      richText.getTextAndCursor,
-      richText.setContentWithTrailingSpace,
+      richText.getPlainTextAndCursor,
+      richText.replacePlainTextRange,
     ],
   );
 
   const applyChannelInsert = React.useCallback(
     (suggestion: ChannelSuggestion) => {
-      const { text, cursor } = richText.getTextAndCursor();
-      const result = channelLinks.insertChannel(suggestion, text, cursor);
-      richText.setContentWithTrailingSpace(result.nextContent);
-      setContent(result.nextContent);
-      contentRef.current = result.nextContent;
+      const { cursor } = richText.getPlainTextAndCursor();
+      const { replaceFromOffset, replaceToOffset, insertText } =
+        channelLinks.insertChannel(suggestion, cursor);
+      richText.replacePlainTextRange(
+        replaceFromOffset,
+        replaceToOffset,
+        insertText,
+      );
     },
     [
       channelLinks.insertChannel,
-      richText.getTextAndCursor,
-      richText.setContentWithTrailingSpace,
+      richText.getPlainTextAndCursor,
+      richText.replacePlainTextRange,
     ],
   );
 
@@ -147,7 +154,7 @@ export function ForumComposer({
   // ── @ mention picker (toolbar button) ───────────────────────────────
   const openMentionPicker = React.useCallback(() => {
     if (!richText.editor) return;
-    const { text, cursor } = richText.getTextAndCursor();
+    const { text, cursor } = richText.getPlainTextAndCursor();
 
     const beforeCursor = text.slice(0, cursor);
     if (/(?:^|[\s])@[^\s]*$/.test(beforeCursor)) {
@@ -162,12 +169,12 @@ export function ForumComposer({
     richText.editor.chain().focus().insertContent(prefix).run();
     setIsEmojiPickerOpen(false);
 
-    const updatedText = richText.editor.state.doc.textContent;
-    const { cursor: updatedCursor } = richText.getTextAndCursor();
+    const { text: updatedText, cursor: updatedCursor } =
+      richText.getPlainTextAndCursor();
     mentions.updateMentionQuery(updatedText, updatedCursor);
   }, [
     richText.editor,
-    richText.getTextAndCursor,
+    richText.getPlainTextAndCursor,
     richText.focus,
     mentions.updateMentionQuery,
   ]);
diff --git a/desktop/src/features/messages/lib/plainTextProjection.test.mjs b/desktop/src/features/messages/lib/plainTextProjection.test.mjs
new file mode 100644
index 000000000..762c56c04
--- /dev/null
+++ b/desktop/src/features/messages/lib/plainTextProjection.test.mjs
@@ -0,0 +1,272 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { getSchema } from "@tiptap/core";
+import StarterKit from "@tiptap/starter-kit";
+
+import { buildPlainTextProjection } from "./plainTextProjection.ts";
+
+// ── Build the actual Tiptap schema we use in the composer ─────────────
+// Matching useRichTextEditor's StarterKit configuration (minus things
+// that don't affect the schema shape). This guarantees the projection
+// is tested against the *real* node names and types.
+
+const schema = getSchema([
+  StarterKit.configure({
+    hardBreak: { keepMarks: true },
+    heading: false,
+    trailingNode: false,
+    link: false,
+  }),
+]);
+
+const para = (...c) => schema.nodes.paragraph.create(null, c);
+const t = (s) => schema.text(s);
+const br = () => schema.nodes.hardBreak.create();
+const li = (...c) => schema.nodes.listItem.create(null, c);
+const ul = (...c) => schema.nodes.bulletList.create(null, c);
+
+function doc(...content) {
+  return schema.nodes.doc.create(null, content);
+}
+
+// ── Helper: assert text equals textBetween(blockSep, leafText="\n") ───
+
+function assertMatchesTextBetween(d, p) {
+  const expected = d.textBetween(0, d.content.size, "\n", "\n");
+  assert.equal(
+    p.text,
+    expected,
+    `projection.text should equal doc.textBetween(0..size, "\\n", "\\n")`,
+  );
+}
+
+// ── Single-paragraph ──────────────────────────────────────────────────
+
+test("single paragraph: text is the paragraph's content", () => {
+  const d = doc(para(t("hello")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "hello");
+  assertMatchesTextBetween(d, p);
+});
+
+test("single paragraph: PM↔text mapping is identity within text node", () => {
+  const d = doc(para(t("hello")));
+  const p = buildPlainTextProjection(d);
+  // PM: para=0, "hello"=1..6
+  assert.equal(p.mapPMToTextOffset(1), 0);
+  assert.equal(p.mapPMToTextOffset(3), 2);
+  assert.equal(p.mapPMToTextOffset(6), 5);
+  assert.equal(p.mapTextOffsetToPM(0), 1);
+  assert.equal(p.mapTextOffsetToPM(2), 3);
+  assert.equal(p.mapTextOffsetToPM(5), 6);
+});
+
+// ── HardBreak ─────────────────────────────────────────────────────────
+
+test("hardBreak contributes a single \\n", () => {
+  const d = doc(para(t("hello"), br(), t("world")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "hello\nworld");
+  assertMatchesTextBetween(d, p);
+});
+
+test("cursor before <br> maps to text offset just before the \\n", () => {
+  // PM: para=0, "hello"=1..5, <br>=6, "world"=7..11
+  const d = doc(para(t("hello"), br(), t("world")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapPMToTextOffset(6), 5);
+});
+
+test("cursor after <br> maps to text offset just after the \\n", () => {
+  const d = doc(para(t("hello"), br(), t("world")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapPMToTextOffset(7), 6);
+});
+
+test("text offset right after \\n maps to PM after the break", () => {
+  const d = doc(para(t("hello"), br(), t("world")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapTextOffsetToPM(6), 7);
+});
+
+test("text offset just before \\n maps to PM before the break", () => {
+  const d = doc(para(t("hello"), br(), t("world")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapTextOffsetToPM(5), 6);
+});
+
+// ── Multi-paragraph ───────────────────────────────────────────────────
+
+test("two paragraphs: block boundary contributes a single \\n", () => {
+  const d = doc(para(t("aaa")), para(t("bbb")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "aaa\nbbb");
+  assertMatchesTextBetween(d, p);
+});
+
+test("two paragraphs: cursor in second paragraph maps past the boundary \\n", () => {
+  // PM: p1 nodeSize=5 (token + 3 chars + token), p2 opens at PM=5
+  //     "aaa" text at 1..4 (size 3), p1 closes at PM=4..5, p2 opens at PM=5,
+  //     "bbb" text at 6..9
+  const d = doc(para(t("aaa")), para(t("bbb")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapPMToTextOffset(6), 4);
+  assert.equal(p.mapTextOffsetToPM(4), 6);
+});
+
+test("three paragraphs: cumulative block boundaries", () => {
+  const d = doc(para(t("aaa")), para(t("bbb")), para(t("ccc")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "aaa\nbbb\nccc");
+  assertMatchesTextBetween(d, p);
+  // "ccc" text starts at PM=11 → text offset 8
+  assert.equal(p.mapPMToTextOffset(11), 8);
+  assert.equal(p.mapTextOffsetToPM(8), 11);
+});
+
+// ── HardBreak + multi-paragraph ──────────────────────────────────────
+
+test("paragraph with <br> then second paragraph", () => {
+  const d = doc(para(t("line1"), br(), t("line2")), para(t("para2")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "line1\nline2\npara2");
+  assertMatchesTextBetween(d, p);
+});
+
+test("range crossing a <br>", () => {
+  // PM: para=0, "line1"=1..5, <br>=6, "line2"=7..11
+  // textOffset 2..8 = "ne1\nli" → PM 3..9
+  const d = doc(para(t("line1"), br(), t("line2")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapTextOffsetToPM(2), 3);
+  assert.equal(p.mapTextOffsetToPM(8), 9);
+});
+
+// ── List items (nested blocks under bulletList) ──────────────────────
+
+test("bullet list: items separated by \\n (matches textBetween)", () => {
+  const d = doc(ul(li(para(t("first"))), li(para(t("second")))));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "first\nsecond");
+  assertMatchesTextBetween(d, p);
+});
+
+test("paragraph + bullet list", () => {
+  const d = doc(para(t("intro")), ul(li(para(t("a"))), li(para(t("b")))));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "intro\na\nb");
+  assertMatchesTextBetween(d, p);
+});
+
+test("list item: PM↔text round-trip lands in the right item", () => {
+  const d = doc(ul(li(para(t("first"))), li(para(t("second")))));
+  const p = buildPlainTextProjection(d);
+  const pm = p.mapTextOffsetToPM(6); // start of "second"
+  assert.equal(p.mapPMToTextOffset(pm), 6);
+});
+
+// ── Edge cases ───────────────────────────────────────────────────────
+
+test("offset 0 maps inside the first text node", () => {
+  const d = doc(para(t("hello")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapTextOffsetToPM(0), 1);
+});
+
+test("offset past end clamps to end-of-doc content position", () => {
+  const d = doc(para(t("hello")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapTextOffsetToPM(999), 6);
+});
+
+test("PM position past doc clamps to text.length", () => {
+  const d = doc(para(t("hello")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapPMToTextOffset(999), 5);
+});
+
+test("empty paragraph: empty text, mappings clamp safely", () => {
+  const d = doc(para());
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "");
+  assert.equal(p.mapPMToTextOffset(0), 0);
+  assert.equal(p.mapPMToTextOffset(1), 0);
+  // Inside the empty paragraph → PM=1
+  assert.equal(p.mapTextOffsetToPM(0), 1);
+});
+
+// ── Empty leaf blocks (paste / draft restore can produce these) ─────
+
+test("empty paragraph after text: trailing \\n preserved", () => {
+  const d = doc(para(t("a")), para());
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "a\n");
+  assertMatchesTextBetween(d, p);
+});
+
+test("empty paragraph after text: offset round-trips into the empty block", () => {
+  // PM: p1=0..2 (size 3: <p>1 + "a"1 + </p>1), text "a" at 1..2
+  //     p2=3..4 (size 2: <p>1 + </p>1), empty interior at PM=4
+  const d = doc(para(t("a")), para());
+  const p = buildPlainTextProjection(d);
+  // offset 2 (right after the `\n`) → PM=4 (inside empty p2) → back to 2
+  assert.equal(p.mapTextOffsetToPM(2), 4);
+  assert.equal(p.mapPMToTextOffset(4), 2);
+});
+
+test("empty paragraph before text: leading \\n preserved", () => {
+  const d = doc(para(), para(t("a")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "\na");
+  assertMatchesTextBetween(d, p);
+});
+
+test("empty paragraph before text: offset 0 lands inside the empty block", () => {
+  // PM: p1=0..1 (size 2: <p>1 + </p>1), empty interior at PM=1
+  //     p2=2..4 (size 3), "a" at PM=3
+  const d = doc(para(), para(t("a")));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.mapTextOffsetToPM(0), 1);
+  assert.equal(p.mapPMToTextOffset(1), 0);
+});
+
+test("two empty paragraphs: single \\n separator, both interiors reachable", () => {
+  const d = doc(para(), para());
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "\n");
+  assertMatchesTextBetween(d, p);
+  // PM: p1=0..1 (size 2), interior=1. p2=2..3 (size 2), interior=3.
+  assert.equal(p.mapTextOffsetToPM(0), 1);
+  assert.equal(p.mapTextOffsetToPM(1), 3);
+  assert.equal(p.mapPMToTextOffset(1), 0);
+  assert.equal(p.mapPMToTextOffset(3), 1);
+});
+
+test("empty list item: interior reachable, separators preserved", () => {
+  const d = doc(ul(li(para()), li(para(t("x")))));
+  const p = buildPlainTextProjection(d);
+  assert.equal(p.text, "\nx");
+  assertMatchesTextBetween(d, p);
+});
+
+// ── Property: round-trip ─────────────────────────────────────────────
+
+test("round-trip: text offset → PM → text offset is identity", () => {
+  const d = doc(
+    para(t("line1"), br(), t("line2")),
+    para(),
+    para(t("para3")),
+    ul(li(para(t("item-a"))), li(para()), li(para(t("item-c")))),
+  );
+  const p = buildPlainTextProjection(d);
+  for (let offset = 0; offset <= p.text.length; offset++) {
+    const pm = p.mapTextOffsetToPM(offset);
+    const back = p.mapPMToTextOffset(pm);
+    assert.equal(
+      back,
+      offset,
+      `offset ${offset} → pm ${pm} → offset ${back} (text=${JSON.stringify(p.text)})`,
+    );
+  }
+});
diff --git a/desktop/src/features/messages/lib/plainTextProjection.ts b/desktop/src/features/messages/lib/plainTextProjection.ts
new file mode 100644
index 000000000..109ae79a7
--- /dev/null
+++ b/desktop/src/features/messages/lib/plainTextProjection.ts
@@ -0,0 +1,237 @@
+import type { Node as ProseMirrorNode } from "@tiptap/pm/model";
+
+/**
+ * Plain-text projection of a ProseMirror document.
+ *
+ * Plain-text is what a textarea-shaped consumer (mention/channel/emoji
+ * autocomplete) reads: hard breaks render as `\n`, and a single `\n`
+ * separates content in different leaf blocks (paragraphs, list items,
+ * etc.) — the same convention as `doc.textBetween(from, to, "\n", "\n")`.
+ *
+ * The same walk is used to map *both* directions:
+ *  - PM position  → plain-text offset (`mapPMToTextOffset`)
+ *  - text offset  → PM position       (`mapTextOffsetToPM`)
+ *
+ * Keeping a single source of truth means the two mappings can't drift,
+ * which is the historic source of off-by-one bugs around `hardBreak` and
+ * multi-block docs.
+ *
+ * Pure function — no editor / view / React dependency.
+ */
+export type PlainTextProjection = {
+  /** The plain-text projection of the document. */
+  text: string;
+  /** Map a ProseMirror position to a plain-text offset. Clamps to [0, text.length]. */
+  mapPMToTextOffset: (pm: number) => number;
+  /** Map a plain-text offset to a ProseMirror position. Clamps to a valid in-doc position. */
+  mapTextOffsetToPM: (offset: number) => number;
+};
+
+type Segment =
+  // A text node: pm range and text range have equal length.
+  | {
+      kind: "text";
+      pmFrom: number;
+      pmTo: number;
+      textFrom: number;
+      textTo: number;
+    }
+  // A hardBreak: 1 PM position wide, contributes one `\n`.
+  | {
+      kind: "hardBreak";
+      pmFrom: number;
+      pmTo: number;
+      textFrom: number;
+      textTo: number;
+    }
+  // A boundary between two leaf-block siblings (paragraphs, list items,
+  // headings, etc.) — zero PM positions wide. Both sides resolve to
+  // `pmAt`, which is the boundary point between the two blocks (= start
+  // of the next leaf-block's content, minus its opening token).
+  | {
+      kind: "blockBoundary";
+      pmAt: number;
+      textFrom: number;
+      textTo: number;
+    }
+  // The interior of an empty leaf block — a zero-width "content slot"
+  // at PM position `pmAt` and text offset `textAt`. Lets cursor / range
+  // endpoints land inside empty paragraphs (paste, draft restore) and
+  // round-trip correctly through the mapping.
+  | {
+      kind: "emptyBlockContent";
+      pmAt: number;
+      textAt: number;
+    };
+
+/**
+ * Build a `PlainTextProjection` for the given doc.
+ *
+ * Walks the doc once and records each text node, hard break, and the
+ * boundary between consecutive leaf-blocks. A "leaf block" is any block
+ * node that does not itself contain blocks — `doc.textBetween` treats
+ * exactly those boundaries as inserting the blockSeparator, and we do
+ * the same so our text projection equals `doc.textBetween(0, end, "\n", "\n")`.
+ */
+export function buildPlainTextProjection(
+  doc: ProseMirrorNode,
+): PlainTextProjection {
+  const segments: Segment[] = [];
+  const textParts: string[] = [];
+  let cursorText = 0;
+  /**
+   * True once we've entered at least one leaf-block. Subsequent leaf-blocks
+   * emit a boundary `\n` before their content — matching `textBetween`.
+   */
+  let sawLeafBlock = false;
+
+  doc.descendants((node, pos) => {
+    // ── Leaf inline: text ──────────────────────────────────────────
+    if (node.isText) {
+      const t = node.text ?? "";
+      segments.push({
+        kind: "text",
+        pmFrom: pos,
+        pmTo: pos + t.length,
+        textFrom: cursorText,
+        textTo: cursorText + t.length,
+      });
+      textParts.push(t);
+      cursorText += t.length;
+      return false; // text nodes have no children
+    }
+
+    // ── Leaf inline: hard break ────────────────────────────────────
+    if (node.type.name === "hardBreak") {
+      segments.push({
+        kind: "hardBreak",
+        pmFrom: pos,
+        pmTo: pos + 1,
+        textFrom: cursorText,
+        textTo: cursorText + 1,
+      });
+      textParts.push("\n");
+      cursorText += 1;
+      return false;
+    }
+
+    // ── Block ──────────────────────────────────────────────────────
+    if (node.isBlock) {
+      // Only "leaf blocks" (those with inline content — paragraphs,
+      // headings, code blocks) produce text in `textBetween`'s sense.
+      // Mixed-content blocks (lists, blockquotes) just contain other
+      // blocks, so their *inner* leaf blocks record boundaries instead.
+      const isLeafBlock = !!node.type.inlineContent;
+
+      if (isLeafBlock) {
+        if (sawLeafBlock) {
+          // Boundary between the previous leaf-block and this one. The
+          // PM position of the boundary is `pos` — the opening token of
+          // the new leaf-block. textBetween emits the separator here.
+          segments.push({
+            kind: "blockBoundary",
+            pmAt: pos,
+            textFrom: cursorText,
+            textTo: cursorText + 1,
+          });
+          textParts.push("\n");
+          cursorText += 1;
+        }
+        sawLeafBlock = true;
+
+        // If the leaf block is empty, no text/hardBreak segment will be
+        // recorded inside it — but we still need a target for the
+        // *interior* PM position so cursor / range endpoints sitting
+        // inside the empty block round-trip correctly. Record a
+        // zero-width content slot at `pos + 1` (= the position right
+        // after this block's opening token).
+        if (node.content.size === 0) {
+          segments.push({
+            kind: "emptyBlockContent",
+            pmAt: pos + 1,
+            textAt: cursorText,
+          });
+        }
+      }
+      return true; // descend into block children
+    }
+
+    // Other inline leaf nodes (none today) — skip silently.
+    return true;
+  });
+
+  const text = textParts.join("");
+
+  function mapPMToTextOffset(pm: number): number {
+    if (pm <= 0) return 0;
+    for (const seg of segments) {
+      if (seg.kind === "text") {
+        if (pm <= seg.pmTo) {
+          if (pm <= seg.pmFrom) return seg.textFrom;
+          return seg.textFrom + (pm - seg.pmFrom);
+        }
+      } else if (seg.kind === "hardBreak") {
+        if (pm <= seg.pmFrom) return seg.textFrom;
+        if (pm <= seg.pmTo) return seg.textTo;
+      } else if (seg.kind === "blockBoundary") {
+        // Zero PM-width.
+        if (pm <= seg.pmAt) return seg.textFrom;
+      } else {
+        // emptyBlockContent — interior of an empty leaf block.
+        if (pm <= seg.pmAt) return seg.textAt;
+      }
+    }
+    return text.length;
+  }
+
+  function mapTextOffsetToPM(offset: number): number {
+    if (offset <= 0) {
+      // Position right inside the first content-carrying location.
+      const first = segments.find(
+        (s) =>
+          s.kind === "text" ||
+          s.kind === "hardBreak" ||
+          s.kind === "emptyBlockContent",
+      );
+      if (first) {
+        return first.kind === "emptyBlockContent" ? first.pmAt : first.pmFrom;
+      }
+      return doc.content.size > 0 ? 1 : 0;
+    }
+    // Iterate segments; an offset that falls *exactly* at the right edge
+    // of a separator segment (hardBreak / blockBoundary) is interpreted
+    // as "start of the next content node" and so falls through to be
+    // claimed by the next segment (a text segment, or an
+    // emptyBlockContent slot if the next leaf-block is empty).
+    for (const seg of segments) {
+      if (seg.kind === "text") {
+        if (offset <= seg.textTo) {
+          return seg.pmFrom + (offset - seg.textFrom);
+        }
+      } else if (seg.kind === "hardBreak") {
+        if (offset < seg.textTo) {
+          // Anywhere before the right edge of the `\n` → before the break.
+          return seg.pmFrom;
+        }
+        // offset === seg.textTo → fall through to the next segment.
+      } else if (seg.kind === "blockBoundary") {
+        // Zero PM-width.
+        // offset <  textTo → "end of previous block" → pmAt
+        // offset === textTo → "start of next block" → fall through.
+        if (offset < seg.textTo) return seg.pmAt;
+      } else {
+        // emptyBlockContent — interior of an empty leaf block.
+        // offset === textAt → land inside the empty block at pmAt.
+        if (offset <= seg.textAt) return seg.pmAt;
+      }
+    }
+    // Beyond all content → end-of-doc text position.
+    const last = segments[segments.length - 1];
+    if (!last) return doc.content.size > 0 ? 1 : 0;
+    if (last.kind === "text" || last.kind === "hardBreak") return last.pmTo;
+    if (last.kind === "emptyBlockContent") return last.pmAt;
+    return last.pmAt;
+  }
+
+  return { text, mapPMToTextOffset, mapTextOffsetToPM };
+}
diff --git a/desktop/src/features/messages/lib/useChannelLinks.ts b/desktop/src/features/messages/lib/useChannelLinks.ts
index 7ecb43f07..c33888863 100644
--- a/desktop/src/features/messages/lib/useChannelLinks.ts
+++ b/desktop/src/features/messages/lib/useChannelLinks.ts
@@ -2,6 +2,7 @@ import * as React from "react";
 
 import { useChannelNavigation } from "@/shared/context/ChannelNavigationContext";
 import { detectPrefixQuery } from "@/shared/lib/detectPrefixQuery";
+import type { AutocompleteEdit } from "./useRichTextEditor";
 
 export type ChannelSuggestion = {
   id: string;
@@ -74,27 +75,23 @@ export function useChannelLinks() {
   const isChannelOpen = channelQuery !== null && channelSuggestions.length > 0;
 
   const insertChannel = React.useCallback(
-    (
-      suggestion: ChannelSuggestion,
-      content: string,
-      selectionEnd: number,
-    ): { nextContent: string; nextCursor: number } => {
+    (suggestion: ChannelSuggestion, selectionEnd: number): AutocompleteEdit => {
       // Cancel any pending debounced detection — user already selected
       if (debounceTimerRef.current !== null) {
         clearTimeout(debounceTimerRef.current);
         debounceTimerRef.current = null;
       }
 
-      const before = content.slice(0, channelStartIndex);
-      const after = content.slice(selectionEnd);
-      const inserted = `#${suggestion.name} `;
-      const nextContent = `${before}${inserted}${after}`;
-      const nextCursor = before.length + inserted.length;
+      const insertText = `#${suggestion.name} `;
 
       setChannelQuery(null);
       setChannelSelectedIndex(0);
 
-      return { nextContent, nextCursor };
+      return {
+        replaceFromOffset: channelStartIndex,
+        replaceToOffset: selectionEnd,
+        insertText,
+      };
     },
     [channelStartIndex],
   );
diff --git a/desktop/src/features/messages/lib/useEmojiAutocomplete.ts b/desktop/src/features/messages/lib/useEmojiAutocomplete.ts
index d655911d2..beedd1f34 100644
--- a/desktop/src/features/messages/lib/useEmojiAutocomplete.ts
+++ b/desktop/src/features/messages/lib/useEmojiAutocomplete.ts
@@ -3,6 +3,8 @@ import * as React from "react";
 import { init, SearchIndex } from "emoji-mart";
 import data from "@emoji-mart/data";
 
+import type { AutocompleteEdit } from "./useRichTextEditor";
+
 export type EmojiSuggestion = {
   id: string;
   name: string;
@@ -101,26 +103,22 @@ export function useEmojiAutocomplete() {
   const isEmojiAutocompleteOpen = emojiQuery !== null && suggestions.length > 0;
 
   const insertEmoji = React.useCallback(
-    (
-      suggestion: EmojiSuggestion,
-      content: string,
-      selectionEnd: number,
-    ): { nextContent: string; nextCursor: number } => {
+    (suggestion: EmojiSuggestion, selectionEnd: number): AutocompleteEdit => {
       if (debounceTimerRef.current !== null) {
         clearTimeout(debounceTimerRef.current);
         debounceTimerRef.current = null;
       }
 
-      const before = content.slice(0, emojiStartIndex);
-      const after = content.slice(selectionEnd);
-      const inserted = `${suggestion.native} `;
-      const nextContent = `${before}${inserted}${after}`;
-      const nextCursor = before.length + inserted.length;
+      const insertText = `${suggestion.native} `;
 
       setEmojiQuery(null);
       setEmojiSelectedIndex(0);
 
-      return { nextContent, nextCursor };
+      return {
+        replaceFromOffset: emojiStartIndex,
+        replaceToOffset: selectionEnd,
+        insertText,
+      };
     },
     [emojiStartIndex],
   );
diff --git a/desktop/src/features/messages/lib/useMentions.ts b/desktop/src/features/messages/lib/useMentions.ts
index 819a8dee3..d09eec76c 100644
--- a/desktop/src/features/messages/lib/useMentions.ts
+++ b/desktop/src/features/messages/lib/useMentions.ts
@@ -6,6 +6,7 @@ import {
 } from "@/features/agents/hooks";
 import { useChannelMembersQuery } from "@/features/channels/hooks";
 import type { MentionSuggestion } from "@/features/messages/ui/MentionAutocomplete";
+import type { AutocompleteEdit } from "./useRichTextEditor";
 import type { ChannelMember } from "@/shared/api/types";
 import type { UserProfileLookup } from "@/features/profile/lib/identity";
 import { detectPrefixQuery } from "@/shared/lib/detectPrefixQuery";
@@ -169,11 +170,7 @@ export function useMentions(
   const isMentionOpen = mentionQuery !== null && suggestions.length > 0;
 
   const insertMention = React.useCallback(
-    (
-      suggestion: MentionSuggestion,
-      content: string,
-      selectionEnd: number,
-    ): { nextContent: string; nextCursor: number } => {
+    (suggestion: MentionSuggestion, selectionEnd: number): AutocompleteEdit => {
       // Cancel any pending debounced detection — user already selected
       if (debounceTimerRef.current !== null) {
         clearTimeout(debounceTimerRef.current);
@@ -181,11 +178,7 @@ export function useMentions(
       }
 
       const displayName = suggestion.displayName;
-      const before = content.slice(0, mentionStartIndex);
-      const after = content.slice(selectionEnd);
-      const inserted = `@${displayName} `;
-      const nextContent = `${before}${inserted}${after}`;
-      const nextCursor = before.length + inserted.length;
+      const insertText = `@${displayName} `;
 
       const mentions = mentionMapRef.current;
       mentions.set(displayName, suggestion.pubkey);
@@ -193,7 +186,11 @@ export function useMentions(
       setMentionQuery(null);
       setMentionSelectedIndex(0);
 
-      return { nextContent, nextCursor };
+      return {
+        replaceFromOffset: mentionStartIndex,
+        replaceToOffset: selectionEnd,
+        insertText,
+      };
     },
     [mentionStartIndex],
   );
diff --git a/desktop/src/features/messages/lib/useRichTextEditor.test.mjs b/desktop/src/features/messages/lib/useRichTextEditor.test.mjs
deleted file mode 100644
index ce53969e1..000000000
--- a/desktop/src/features/messages/lib/useRichTextEditor.test.mjs
+++ /dev/null
@@ -1,126 +0,0 @@
-import assert from "node:assert/strict";
-import test from "node:test";
-
-/**
- * Pure extraction of the ProseMirror → plain-text cursor mapping logic
- * from getTextAndCursor in useRichTextEditor.ts.
- *
- * Takes a list of "visited nodes" (as the descendants callback would see them)
- * and a ProseMirror anchor position, returns the plain-text offset.
- */
-function mapAnchorToPlainText(nodes, anchor) {
-  let offset = 0;
-  let found = false;
-  for (const { isText, isBlock, pos, nodeSize } of nodes) {
-    if (found) break;
-    if (isText) {
-      const nodeEnd = pos + nodeSize;
-      if (anchor <= nodeEnd) {
-        offset += anchor - pos;
-        found = true;
-        break;
-      }
-      offset += nodeSize;
-    } else if (isBlock && pos > 0) {
-      offset += 1;
-    }
-  }
-  return found ? offset : -1; // -1 means "fell through"
-}
-
-// ── Single paragraph ──────────────────────────────────────────────────
-
-test("cursor at start of single paragraph", () => {
-  // doc > paragraph(pos=0) > text "hello"(pos=1, size=5)
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 7 },
-    { isText: true, isBlock: false, pos: 1, nodeSize: 5 },
-  ];
-  // Anchor at pos=1 → plain-text offset 0
-  assert.equal(mapAnchorToPlainText(nodes, 1), 0);
-});
-
-test("cursor at end of single paragraph", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 7 },
-    { isText: true, isBlock: false, pos: 1, nodeSize: 5 },
-  ];
-  // Anchor at pos=6 → plain-text offset 5
-  assert.equal(mapAnchorToPlainText(nodes, 6), 5);
-});
-
-test("cursor mid-word in single paragraph", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 7 },
-    { isText: true, isBlock: false, pos: 1, nodeSize: 5 },
-  ];
-  // Anchor at pos=3 → plain-text offset 2 (after "he")
-  assert.equal(mapAnchorToPlainText(nodes, 3), 2);
-});
-
-// ── Two paragraphs (the bug scenario) ─────────────────────────────────
-// doc structure: doc > p1("hello") > p2("world")
-// ProseMirror positions: doc=0, p1=0, "hello"=1..5, /p1=6, p2=7, "world"=8..12, /p2=13
-// textContent = "hello\nworld" (11 chars)
-
-test("cursor in second paragraph accounts for block boundary newline", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 7 }, // p1
-    { isText: true, isBlock: false, pos: 1, nodeSize: 5 }, // "hello"
-    { isText: false, isBlock: true, pos: 7, nodeSize: 7 }, // p2 (pos > 0 → newline)
-    { isText: true, isBlock: false, pos: 8, nodeSize: 5 }, // "world"
-  ];
-  // Anchor at pos=8 → start of "world" → plain-text offset 6 ("hello\n" = 6 chars)
-  assert.equal(mapAnchorToPlainText(nodes, 8), 6);
-});
-
-test("cursor mid-word in second paragraph", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 7 },
-    { isText: true, isBlock: false, pos: 1, nodeSize: 5 },
-    { isText: false, isBlock: true, pos: 7, nodeSize: 7 },
-    { isText: true, isBlock: false, pos: 8, nodeSize: 5 },
-  ];
-  // Anchor at pos=10 → "wo|rld" → plain-text offset 8 ("hello\nwo" = 8 chars)
-  assert.equal(mapAnchorToPlainText(nodes, 10), 8);
-});
-
-// ── Three paragraphs (cumulative drift) ───────────────────────────────
-// "aaa\nbbb\nccc" — without the fix, offset would drift by 1 per boundary
-
-test("cursor in third paragraph accounts for two block boundaries", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 5 }, // p1
-    { isText: true, isBlock: false, pos: 1, nodeSize: 3 }, // "aaa"
-    { isText: false, isBlock: true, pos: 5, nodeSize: 5 }, // p2
-    { isText: true, isBlock: false, pos: 6, nodeSize: 3 }, // "bbb"
-    { isText: false, isBlock: true, pos: 10, nodeSize: 5 }, // p3
-    { isText: true, isBlock: false, pos: 11, nodeSize: 3 }, // "ccc"
-  ];
-  // Anchor at pos=11 → start of "ccc" → plain-text offset 8 ("aaa\nbbb\n" = 8 chars)
-  assert.equal(mapAnchorToPlainText(nodes, 11), 8);
-});
-
-test("cursor at end of third paragraph", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 5 },
-    { isText: true, isBlock: false, pos: 1, nodeSize: 3 },
-    { isText: false, isBlock: true, pos: 5, nodeSize: 5 },
-    { isText: true, isBlock: false, pos: 6, nodeSize: 3 },
-    { isText: false, isBlock: true, pos: 10, nodeSize: 5 },
-    { isText: true, isBlock: false, pos: 11, nodeSize: 3 },
-  ];
-  // Anchor at pos=14 → end of "ccc" → plain-text offset 11 ("aaa\nbbb\nccc" = 11 chars)
-  assert.equal(mapAnchorToPlainText(nodes, 14), 11);
-});
-
-// ── First paragraph is unaffected (pos === 0, no newline) ─────────────
-
-test("first block boundary at pos 0 does not add newline", () => {
-  const nodes = [
-    { isText: false, isBlock: true, pos: 0, nodeSize: 7 },
-    { isText: true, isBlock: false, pos: 1, nodeSize: 5 },
-  ];
-  // Anchor at pos=4 → plain-text offset 3 (no extra newline for first block)
-  assert.equal(mapAnchorToPlainText(nodes, 4), 3);
-});
diff --git a/desktop/src/features/messages/lib/useRichTextEditor.ts b/desktop/src/features/messages/lib/useRichTextEditor.ts
index d04720e30..3c0c4399c 100644
--- a/desktop/src/features/messages/lib/useRichTextEditor.ts
+++ b/desktop/src/features/messages/lib/useRichTextEditor.ts
@@ -14,6 +14,18 @@ import {
   MentionHighlightExtension,
   mentionHighlightKey,
 } from "./mentionHighlightExtension";
+import { buildPlainTextProjection } from "./plainTextProjection";
+
+/**
+ * Plain-text edit descriptor returned by autocomplete hooks
+ * (mentions / channel links / emoji). Offsets are in plain-text space —
+ * see `buildPlainTextProjection`.
+ */
+export type AutocompleteEdit = {
+  replaceFromOffset: number;
+  replaceToOffset: number;
+  insertText: string;
+};
 
 export type RichTextEditorOptions = {
   placeholder?: string;
@@ -262,7 +274,11 @@ export function useRichTextEditor({
       },
       onUpdate: ({ editor: ed }) => {
         const markdown = getMarkdownFromEditor(ed);
-        const text = ed.state.doc.textContent;
+        // Use the same plain-text projection that `getPlainTextAndCursor`
+        // uses, so autocomplete detection sees the *same* string the
+        // cursor offset is mapped against. `state.doc.textContent` would
+        // diverge by 1 per hard-break / block boundary.
+        const text = buildPlainTextProjection(ed.state.doc).text;
         onUpdateRef.current?.({ markdown, text });
       },
     },
@@ -332,86 +348,76 @@ export function useRichTextEditor({
   }, [editor]);
 
   /**
-   * Replace editor content and append a trailing space that survives parsing.
+   * Plain-text view of the document plus the cursor position in
+   * plain-text offset space. Used by autocomplete detection (mentions,
+   * channel links, emoji) which is shaped like a textarea.
+   *
+   * The plain-text projection treats both `hardBreak` and inter-block
+   * boundaries as `\n` — matching `doc.textBetween(0, end, "\n", "\n")`.
+   * See `plainTextProjection.ts`.
+   */
+  const getPlainTextAndCursor = React.useCallback((): {
+    text: string;
+    cursor: number;
+  } => {
+    if (!editor) return { text: "", cursor: 0 };
+    const projection = buildPlainTextProjection(editor.state.doc);
+    const anchor = editor.state.selection.anchor;
+    return {
+      text: projection.text,
+      cursor: projection.mapPMToTextOffset(anchor),
+    };
+  }, [editor]);
+
+  /**
+   * Replace a plain-text range with literal text, in a single native
+   * ProseMirror transaction.
+   *
+   * `fromOffset` and `toOffset` are in plain-text-offset space (the
+   * same space as `getPlainTextAndCursor`). `text` is inserted verbatim
+   * — including any trailing space — without a markdown re-parse.
    *
-   * `setContent(markdown)` roundtrips through TipTap's markdown parser which
-   * strips trailing whitespace from text nodes. TipTap's `insertContent(" ")`
-   * also normalises it away. This method bypasses both by creating a raw
-   * ProseMirror text node and inserting it via a direct transaction — the
-   * only path that reliably preserves a literal trailing space.
+   * This replaces the old `setContentWithTrailingSpace` + full-doc
+   * markdown round-trip used by autocomplete: by going through
+   * `tr.insertText` we preserve active marks, hard breaks, list
+   * structure, undo history continuity, and any whitespace.
    *
-   * Used by mention and channel-link autocomplete insertion.
+   * Returns the new cursor PM position, mapped through `tr.mapping` so
+   * callers get a position that's valid after the transaction is
+   * applied.
    */
-  const setContentWithTrailingSpace = React.useCallback(
-    (markdown: string) => {
+  const replacePlainTextRange = React.useCallback(
+    (fromOffset: number, toOffset: number, text: string) => {
       if (!editor) return;
-      editor.commands.setContent(markdown);
-      // Insert a literal space via a raw ProseMirror transaction so it
-      // bypasses TipTap's content parser which strips trailing whitespace.
-      const { tr, schema, doc } = editor.state;
-      const endPos = doc.content.size - 1; // before the closing node token
-      const spaceNode = schema.text(" ");
-      tr.insert(endPos, spaceNode);
-      // Place cursor after the inserted space.
-      const cursorPos = endPos + spaceNode.nodeSize;
-      tr.setSelection(TextSelection.create(tr.doc, cursorPos));
+      const projection = buildPlainTextProjection(editor.state.doc);
+      const fromPM = projection.mapTextOffsetToPM(fromOffset);
+      const toPM = projection.mapTextOffsetToPM(toOffset);
+
+      const tr = editor.state.tr.insertText(text, fromPM, toPM);
+      // Place cursor at the end of the inserted text. We map `toPM` (the
+      // right end of the replaced range) through the transaction's
+      // mapping — that's the post-transaction position right after the
+      // inserted text, valid even if mark normalisation shifted things.
+      // (Mapping `fromPM + text.length` directly would be a pre-image
+      // position that may not exist in the original doc, which throws
+      // "Position N out of range".)
+      const cursorPM = tr.mapping.map(toPM);
+      tr.setSelection(TextSelection.create(tr.doc, cursorPM));
       editor.view.dispatch(tr);
       editor.view.focus();
     },
     [editor],
   );
 
-  /**
-   * Returns the plain-text content and an approximate cursor offset.
-   * Used to bridge the existing useMentions / useChannelLinks hooks which
-   * were designed for a plain <textarea>.
-   */
-  const getTextAndCursor = React.useCallback((): {
-    text: string;
-    cursor: number;
-  } => {
-    if (!editor) return { text: "", cursor: 0 };
-
-    const { state } = editor;
-    const text = state.doc.textContent;
-    // Map ProseMirror position → plain-text offset.
-    // Walk through text nodes and accumulate length until we pass the anchor.
-    const anchor = state.selection.anchor;
-    let offset = 0;
-    let found = false;
-    state.doc.descendants((node, pos) => {
-      if (found) return false;
-      if (node.isText) {
-        const nodeEnd = pos + node.nodeSize;
-        if (anchor <= nodeEnd) {
-          offset += anchor - pos;
-          found = true;
-          return false;
-        }
-        offset += node.nodeSize;
-      } else if (node.isBlock && pos > 0) {
-        // Block boundaries add a newline in textContent
-        // (but only between blocks, not at the very start)
-        offset += 1;
-      }
-      return undefined;
-    });
-    if (!found) {
-      offset = text.length;
-    }
-
-    return { text, cursor: offset };
-  }, [editor]);
-
   return {
     editor,
     getMarkdown,
     isEmpty,
     clearContent,
     setContent,
-    setContentWithTrailingSpace,
     focus,
-    getTextAndCursor,
+    getPlainTextAndCursor,
+    replacePlainTextRange,
   };
 }
 
diff --git a/desktop/src/features/messages/ui/MessageComposer.tsx b/desktop/src/features/messages/ui/MessageComposer.tsx
index 92ecfab6c..1cc33f39c 100644
--- a/desktop/src/features/messages/ui/MessageComposer.tsx
+++ b/desktop/src/features/messages/ui/MessageComposer.tsx
@@ -19,7 +19,10 @@ import {
   hasMentionClipboardHtml,
   normalizeMentionClipboardHtml,
 } from "@/features/messages/lib/normalizeMentionClipboard";
-import { useRichTextEditor } from "@/features/messages/lib/useRichTextEditor";
+import {
+  type AutocompleteEdit,
+  useRichTextEditor,
+} from "@/features/messages/lib/useRichTextEditor";
 import { useTypingBroadcast } from "@/features/messages/useTypingBroadcast";
 import { cn } from "@/shared/lib/cn";
 import { Button } from "@/shared/ui/button";
@@ -153,8 +156,10 @@ export function MessageComposer({
       setContent(markdown);
       contentRef.current = markdown;
 
-      // Bridge to existing mention/channel/emoji detection hooks.
-      const { cursor } = richText.getTextAndCursor();
+      // Bridge to mention/channel/emoji detection hooks. `text` and the
+      // cursor are both in plain-text-offset space (treating hardBreak
+      // and inter-block boundaries as `\n`).
+      const { cursor } = richText.getPlainTextAndCursor();
       mentions.updateMentionQuery(text, cursor);
       channelLinks.updateChannelQuery(text, cursor);
       emojiAutocomplete.updateEmojiQuery(text, cursor);
@@ -226,51 +231,53 @@ export function MessageComposer({
   // ── Autofocus on mount / channel switch ─────────────────────────────
   useComposerAutofocus(richText.focus, effectiveDraftKey, disabled);
 
-  // ── Mention / channel autocomplete insertion ────────────────────────
+  // ── Mention / channel / emoji autocomplete insertion ────────────────
+  // Hooks return a plain-text edit descriptor; `replacePlainTextRange`
+  // applies it as a single ProseMirror transaction (no markdown round-trip).
+  const applyAutocompleteEdit = React.useCallback(
+    (edit: AutocompleteEdit) => {
+      richText.replacePlainTextRange(
+        edit.replaceFromOffset,
+        edit.replaceToOffset,
+        edit.insertText,
+      );
+    },
+    [richText.replacePlainTextRange],
+  );
+
   const applyMentionInsert = React.useCallback(
     (suggestion: MentionSuggestion) => {
-      const { text, cursor } = richText.getTextAndCursor();
-      const result = mentions.insertMention(suggestion, text, cursor);
-      // setContentWithTrailingSpace re-injects a space after the markdown
-      // roundtrip so the cursor lands ready for the next word.
-      richText.setContentWithTrailingSpace(result.nextContent);
-      setContent(result.nextContent);
-      contentRef.current = result.nextContent;
+      const { cursor } = richText.getPlainTextAndCursor();
+      applyAutocompleteEdit(mentions.insertMention(suggestion, cursor));
     },
     [
+      applyAutocompleteEdit,
       mentions.insertMention,
-      richText.getTextAndCursor,
-      richText.setContentWithTrailingSpace,
+      richText.getPlainTextAndCursor,
     ],
   );
 
   const applyChannelInsert = React.useCallback(
     (suggestion: ChannelSuggestion) => {
-      const { text, cursor } = richText.getTextAndCursor();
-      const result = channelLinks.insertChannel(suggestion, text, cursor);
-      richText.setContentWithTrailingSpace(result.nextContent);
-      setContent(result.nextContent);
-      contentRef.current = result.nextContent;
+      const { cursor } = richText.getPlainTextAndCursor();
+      applyAutocompleteEdit(channelLinks.insertChannel(suggestion, cursor));
     },
     [
+      applyAutocompleteEdit,
       channelLinks.insertChannel,
-      richText.getTextAndCursor,
-      richText.setContentWithTrailingSpace,
+      richText.getPlainTextAndCursor,
     ],
   );
 
   const applyEmojiInsert = React.useCallback(
     (suggestion: EmojiSuggestion) => {
-      const { text, cursor } = richText.getTextAndCursor();
-      const result = emojiAutocomplete.insertEmoji(suggestion, text, cursor);
-      richText.setContentWithTrailingSpace(result.nextContent);
-      setContent(result.nextContent);
-      contentRef.current = result.nextContent;
+      const { cursor } = richText.getPlainTextAndCursor();
+      applyAutocompleteEdit(emojiAutocomplete.insertEmoji(suggestion, cursor));
     },
     [
+      applyAutocompleteEdit,
       emojiAutocomplete.insertEmoji,
-      richText.getTextAndCursor,
-      richText.setContentWithTrailingSpace,
+      richText.getPlainTextAndCursor,
     ],
   );
 
@@ -288,7 +295,7 @@ export function MessageComposer({
   // ── @ mention picker (toolbar button) ───────────────────────────────
   const openMentionPicker = React.useCallback(() => {
     if (!richText.editor) return;
-    const { text, cursor } = richText.getTextAndCursor();
+    const { text, cursor } = richText.getPlainTextAndCursor();
 
     // Check if there's already an @-query in progress
     const beforeCursor = text.slice(0, cursor);
@@ -306,12 +313,12 @@ export function MessageComposer({
     setIsEmojiPickerOpen(false);
 
     // Trigger mention detection after inserting @
-    const updatedText = richText.editor.state.doc.textContent;
-    const { cursor: updatedCursor } = richText.getTextAndCursor();
+    const { text: updatedText, cursor: updatedCursor } =
+      richText.getPlainTextAndCursor();
     mentions.updateMentionQuery(updatedText, updatedCursor);
   }, [
     richText.editor,
-    richText.getTextAndCursor,
+    richText.getPlainTextAndCursor,
     richText.focus,
     mentions.updateMentionQuery,
   ]);
diff --git a/desktop/tests/e2e/mentions.spec.ts b/desktop/tests/e2e/mentions.spec.ts
index 8b0167d66..246581dd2 100644
--- a/desktop/tests/e2e/mentions.spec.ts
+++ b/desktop/tests/e2e/mentions.spec.ts
@@ -74,6 +74,34 @@ test("mention button opens autocomplete and inserts a selected member", async ({
   await expect(input).toHaveText("Hey @alice ");
 });
 
+test("inserting a mention preserves Shift+Enter newlines (regression: bug #2)", async ({
+  page,
+}) => {
+  // Before PR #618, mention insertion round-tripped through
+  // `setContent(markdown)`, which collapsed every Shift+Enter hard
+  // break to a single space. After the fix, autocomplete uses a
+  // native ProseMirror `tr.insertText` transaction and the line
+  // breaks survive.
+  await page.goto("/");
+  await page.getByTestId("channel-general").click();
+  await expect(page.getByTestId("chat-title")).toHaveText("general");
+
+  const input = page.getByTestId("message-input");
+  await input.click();
+  await page.keyboard.type("line one");
+  await page.keyboard.press("Shift+Enter");
+  await page.keyboard.type("line two @ali");
+
+  const dropdown = autocomplete(page);
+  await expect(dropdown.getByText("alice")).toBeVisible();
+  await dropdown.getByText("alice").click();
+
+  // Both lines must still be present, separated by a real line break
+  // (rendered as a `<br>` by Tiptap; the projection sees `\n`).
+  await expect(input).toHaveText(/line one[\s\S]*line two @alice/);
+  await expect(input.locator("br")).toHaveCount(1);
+});
+
 test("keyboard navigation selects mention with Enter", async ({ page }) => {
   await page.goto("/");
   await page.getByTestId("channel-general").click();