fix(desktop): preserve agent mention separator after send (#5623)

**Category:** fix
**User Impact:** Typing immediately after sending to a persistently
addressed agent now continues after the agent mention instead of
corrupting it.

**Problem:** Post-send restoration passed the persistent `@Agent `
prefix through the Markdown parser, which discarded its trailing
separator and left WebKit rendering the caret at the mention boundary.

**Solution:** Restore the prefix as literal ProseMirror text, preserve
the separator, and focus a selection placed at the restored document
end. This does not expand or otherwise change the setting’s existing
scope: persistent addressed agents remain thread-only.

<details>
<summary>File changes</summary>

**desktop/src/features/messages/lib/useRichTextEditor.ts**
Adds a focused plain-text restoration helper that preserves trailing
whitespace while suppressing authored-update reconciliation.

**desktop/src/features/messages/ui/useMentionSendFlow.ts**
Routes non-empty post-send persistent audience restoration through the
literal-text helper instead of Markdown content loading.

**desktop/tests/e2e/persistent-agent-audience.spec.ts**
Extends the real Enter-send flow to assert the preserved separator,
document-end selection, and immediate typing outside the agent mention.

</details>

## Reproduction steps

1. Open a thread with a persistently addressed agent.
2. Send a message with Enter.
3. Confirm the composer restores the addressed agent and a trailing
space.
4. Type immediately without clicking the composer.
5. Confirm the new text appears after the agent mention and the mention
remains highlighted.



https://github.com/user-attachments/assets/92f088aa-a516-48d1-acde-35e29f558f14

---------

Signed-off-by: Taylor Ho <taylorkmho@gmail.com>

desktop/src/features/messages/lib/useRichTextEditor.ts

@@ -742,17 +742,26 @@ export function useRichTextEditor({
     [editor],
   );
 
-  const setContentAndFocusEnd = React.useCallback(
-    (markdown: string) => {
+  /**
+   * Replace the editor document with literal plain text and focus its end.
+   *
+   * Unlike markdown `setContent`, this preserves trailing whitespace. The
+   * transaction is marked as programmatic so authored-update observers do not
+   * reconcile against the intermediate post-send restoration.
+   */
+  const restorePlainTextAndFocusEnd = React.useCallback(
+    (text: string) => {
       if (!editor) return;
-      // The caller already synchronizes composer state. Keep this programmatic
-      // restoration out of user-edit observers (autocomplete/reconciliation),
-      // then move selection in the same command chain.
-      editor
-        .chain()
-        .setContent(markdown, { emitUpdate: false })
-        .focus("end")
-        .run();
+      const paragraph = editor.schema.nodes.paragraph.create(
+        null,
+        text ? editor.schema.text(text) : undefined,
+      );
+      const tr = editor.state.tr
+        .replaceWith(0, editor.state.doc.content.size, paragraph)
+        .setMeta("preventUpdate", true);
+      tr.setSelection(TextSelection.atEnd(tr.doc));
+      editor.view.dispatch(tr);
+      editor.view.focus();
     },
     [editor],
   );
@@ -950,7 +959,7 @@ export function useRichTextEditor({
     isEmpty,
     clearContent,
     setContent,
-    setContentAndFocusEnd,
+    restorePlainTextAndFocusEnd,
     focus,
     focusEnd,
     focusPreserve,

desktop/src/features/messages/ui/useMentionSendFlow.ts

@@ -73,7 +73,7 @@ type UseMentionSendFlowOptions = {
   >;
   richText: Pick<
     UseRichTextEditorResult,
-    "clearContent" | "setContent" | "setContentAndFocusEnd"
+    "clearContent" | "setContent" | "restorePlainTextAndFocusEnd"
   >;
   setContent: (content: string) => void;
   setIsEmojiPickerOpen: React.Dispatch<React.SetStateAction<boolean>>;
@@ -334,7 +334,7 @@ export function useMentionSendFlow({
       setContent(postSendContent);
       contentRef.current = postSendContent;
       if (postSendContent) {
-        richText.setContentAndFocusEnd(postSendContent);
+        richText.restorePlainTextAndFocusEnd(postSendContent);
         mentions.cancelMentionAutocomplete();
       } else richText.clearContent();
       setPendingImeta([]);
@@ -352,7 +352,7 @@ export function useMentionSendFlow({
       mentions.cancelMentionAutocomplete,
       mentions.clearMentions,
       richText.clearContent,
-      richText.setContentAndFocusEnd,
+      richText.restorePlainTextAndFocusEnd,
       setContent,
       setIsEmojiPickerOpen,
       setPendingImeta,

desktop/tests/e2e/persistent-agent-audience.spec.ts

@@ -179,11 +179,24 @@ test("persistent agents transition atomically before Enter-send resolves", async
         // to ProseMirror coordinates proves selection.from/to === doc.content.size.
         return {
           empty: selection.isCollapsed,
+          text: element.textContent,
+          endsWithSpace: element.textContent?.endsWith(" ") ?? false,
           atDocumentEnd: position + 1 === viewDesc.size - 2,
         };
       }),
     )
-    .toEqual({ empty: true, atDocumentEnd: true });
+    .toEqual({
+      empty: true,
+      text: "@Morgarita ",
+      endsWithSpace: true,
+      atDocumentEnd: true,
+    });
+
+  // Exercise the reported contract, not just its selection prerequisites:
+  // immediate typing after restoration must remain outside the mention chip.
+  await input.pressSequentially("testing");
+  await expect(input).toHaveText("@Morgarita testing");
+  await expect(input.locator(".agent-mention-highlight")).toHaveCount(1);
 });
 
 test("timeline agent send remains one-shot and returns to the placeholder", async ({