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

tests/unit/test_chat_persistence.py

@@ -234,8 +234,9 @@ def test_restore_renders_collapsed_thinking_block() -> None:
 
 def test_thinking_block_css_uses_phase08_tokens() -> None:
     """Phase 17 styling (Phase-08 tokens, WCAG AA): the block frame, the
-    ≥44px summary control (brand-ink ≈8.7:1 on surface) and the scrollable
-    scratchpad (ink-soft ≈6.9:1 on surface, 320px cap)."""
+    ≥44px summary control (brand-ink ≈8.7:1 on surface) and the live-tail
+    scratchpad (ink-soft ≈6.9:1 on surface, 320px cap; phase 21 removed the
+    user scroll — owner choice 2026-08-24)."""
     css = _css()
     block = re.search(r"details\.thinking \{([\s\S]*?)\n\}", css)
     assert block, "styles.css must style details.thinking"
@@ -251,8 +252,10 @@ def test_thinking_block_css_uses_phase08_tokens() -> None:
     assert "var(--brand-ink)" in sbody
     assert "cursor: pointer" in sbody
     text = re.search(r"details\.thinking \.thinking-text \{([\s\S]*?)\n\}", css)
-    assert text, "the .thinking-text scroll area must be styled"
+    assert text, "the .thinking-text live-tail area must be styled"
     tbody = text.group(1)
     assert "var(--ink-soft)" in tbody
     assert "max-height: 320px" in tbody
-    assert "overflow-y: auto" in tbody
+    # Phase 21: no user scroll back — the window is a live tail only.
+    assert "overflow-y: hidden" in tbody
+    assert "overflow-y: auto" not in tbody