Scroll just-submitted comment to viewport top with bottom spacer

After a reply is sent, find the new user message via its anchorId and
scrollIntoView with block:start so the submitted comment lands at the
top of the viewport — giving space below to watch the response stream
in. Also render a 40dvh spacer below the thread so that scroll is
available even right after posting.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/IssueChatThread.test.tsx

@@ -881,8 +881,6 @@ describe("IssueChatThread", () => {
 
     const spacer = container.querySelector('[data-testid="issue-chat-bottom-spacer"]');
     expect(spacer).toBeNull();
-=======
->>>>>>> Tune floating reply composer padding, offset, and initial height
 
     act(() => {
       root.unmount();

ui/src/components/IssueChatThread.tsx

@@ -2555,7 +2555,6 @@ export function IssueChatThread({
       spacerInitialReserveRef.current = 0;
     }
   }, [messages]);
-
   useLayoutEffect(() => {
     const composerElement = composerViewportAnchorRef.current;
     if (preserveComposerViewportRef.current) {