fix(ui): thinking window no longer scrolls — live 320px view pinned to the stream tail

tests/e2e/test_thinking_display.py

@@ -18,10 +18,11 @@ Test → story mapping (Playwright Mapping Rule):
 5. ``test_thinking_with_deflection``
 
 Determinism note: the mock paces every SSE frame at 0.02s and the thinking
-text is ~700–900 chars (≈ 60–75 frames ≈ 1.2–1.5s) before the first
-content frame, so "attach → assert open" runs well inside the open window
-on headless Chromium; all other assertions are made after the send button
-re-enables (fully settled state).
+text is ~2 700 chars (≈ 230 frames ≈ 4.5s — lengthened in phase 21 so the
+scratchpad overflows the 320px window) before the first content frame, so
+"attach → assert open" runs well inside the open window on headless
+Chromium; all other assertions are made after the send button re-enables
+(fully settled state).
 """
 from __future__ import annotations
 
@@ -139,8 +140,8 @@ def test_thinking_block_streams_open_then_collapses(
     # token — and is created OPEN.
     details = page.locator(".msg.brain").last.locator("details.thinking")
     details.wait_for(state="attached", timeout=10_000)
-    # The ~800-char thinking stream (≈1.3s) keeps the block open right
-    # after attach — assert while it is still streaming.
+    # The ~2 700-char thinking stream (≈4.5s, phase 21) keeps the block
+    # open right after attach — assert while it is still streaming.
     expect(details).to_have_attribute("open", "")
     expect(details.locator(".thinking-text")).not_to_have_text("")