Clear the composer draft when entering the New Chat / welcome state (#1408)

Clicking "New chat" (the brand/welcome navigation path) left the previous
session's unsent draft in the composer (issue #1343). The direct model-picker
path (createDirectChat) already cleared it, but the welcome path did not.

Clear `#message` in chatRenderer.showWelcomeScreen() — the shared entry point
for that state — resetting its autosized height and dispatching an `input` event
so the send button / autosize listeners update. Switching between existing
sessions loads them directly and does not call showWelcomeScreen, so genuine
drafts are not erased.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_new_chat_clears_input.py

@@ -0,0 +1,34 @@
+"""Regression guard for issue #1343 — clicking "New chat" left the previous
+session's draft text in the composer.
+
+The direct model-picker path (sessions.js:createDirectChat) already cleared the
+input, but the brand/welcome New-Chat navigation path did not. The shared entry
+point for that state is chatRenderer.js:showWelcomeScreen(), which now clears the
+`#message` composer. Switching between existing sessions loads them directly and
+does not call showWelcomeScreen, so real drafts aren't erased.
+
+chatRenderer.js pulls in browser globals, so it can't be imported under node;
+this guards the fix at the source level so it can't be silently dropped.
+"""
+import re
+from pathlib import Path
+
+SRC = (Path(__file__).resolve().parent.parent / "static/js/compare").parent / "chatRenderer.js"
+
+
+def _show_welcome_body() -> str:
+    text = SRC.read_text(encoding="utf-8")
+    start = text.index("export function showWelcomeScreen()")
+    # Body runs until the next top-level `export function` / `function ` decl.
+    rest = text[start + len("export function showWelcomeScreen()"):]
+    m = re.search(r"\nexport function |\nfunction ", rest)
+    return rest[: m.start()] if m else rest
+
+
+def test_new_chat_welcome_clears_the_composer():
+    body = _show_welcome_body()
+    # Clears the draft value...
+    assert re.search(r"getElementById\(['\"]message['\"]\)", body)
+    assert re.search(r"\.value\s*=\s*['\"]['\"]", body), "must reset #message value"
+    # ...and notifies listeners (send button icon / autosize) of the change.
+    assert "new Event('input'" in body or 'new Event("input"' in body