fix(web): open citation comments with the caret at the end

apps/web/src/components/chat/AssistantCitationChip.browser.test.tsx

@@ -1,5 +1,6 @@
 import {
   ASSISTANT_CITATION_MAX_COMMENT_LENGTH,
+  type AssistantCitation,
   EnvironmentId,
   MessageId,
   ThreadId,
@@ -50,12 +51,16 @@ function setup() {
   return { sourceAnchor, [Symbol.dispose]: () => source.remove() };
 }
 
-function mount(sourceAnchor: AssistantCitationSourceAnchor, onSave: () => boolean) {
+function mount(
+  sourceAnchor: AssistantCitationSourceAnchor | undefined,
+  onSave: () => boolean,
+  chipCitation: AssistantCitation = citation,
+) {
   function Composer() {
     const [open, setOpen] = useState(true);
     return (
       <AssistantCitationChip
-        citation={citation}
+        citation={chipCitation}
         composer
         commentEditor={{ open, sourceAnchor, onOpenChange: setOpen, onSave }}
       />
@@ -128,6 +133,25 @@ describe("citation comment source disappearance", () => {
   });
 });
 
+describe("citation comment caret", () => {
+  it("opens a saved comment with the caret after its end, scrolled into view", async () => {
+    using _source = setup();
+    const comment = `${"line\n".repeat(40)}end`;
+    await using _screen = await mount(
+      undefined,
+      vi.fn(() => true),
+      { ...citation, comment },
+    );
+    await vi.waitFor(() => expect(document.activeElement).toBe(textarea()));
+    const input = textarea()!;
+
+    expect(input.scrollHeight).toBeGreaterThan(input.clientHeight);
+    expect(input.scrollTop).toBe(input.scrollHeight - input.clientHeight);
+    await userEvent.keyboard("!");
+    expect(input.value).toBe(`${comment}!`);
+  });
+});
+
 describe("citation comment popup position", () => {
   it("stays on screen when the cited text ends in a code block's hidden overflow", async () => {
     using _source = setup();

apps/web/src/components/chat/AssistantCitationChip.tsx

@@ -190,7 +190,13 @@ export function AssistantCitationChip({
               align="end"
               anchor={popupAnchor}
               initialFocus={() => {
-                commentInputRef.current?.focus({ preventScroll: true });
+                const input = commentInputRef.current;
+                if (input) {
+                  input.focus({ preventScroll: true });
+                  input.setSelectionRange(input.value.length, input.value.length);
+                  // Moving the caret doesn't scroll a long comment to it.
+                  input.scrollTop = input.scrollHeight;
+                }
                 return false;
               }}
               finalFocus={