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

tests/unit/test_thinking_no_scroll.py

@@ -0,0 +1,95 @@
+"""Unit: the "no scroll back" contract for the Thinking window (phase 21,
+owner choice 2026-08-24, roadmap A2).
+
+The live Thinking block is a scratchpad, not a transcript: the 320px window
+always shows the *live tail* of the reasoning stream. The whole functional
+change is one CSS property — ``details.thinking .thinking-text`` goes from
+``overflow-y: auto`` (a user-scrollable window) to ``overflow-y: hidden``
+(a live-tail clip). ``overflow: hidden`` still permits *programmatic*
+scrolling, so the phase-17 JS bottom-pin
+(``textEl.scrollTop = textEl.scrollHeight`` on every thinking chunk) is the
+sole scroller — wheel, drag, and keyboard scrolling stop working.
+
+The browser behavior itself is E2E-covered (tests/e2e/test_thinking_no_scroll.py);
+here we pin the CSS value + the owner-choice comment and the intact
+bottom-pin so a silent regression (``overflow-y`` back to ``auto``, pin
+removed) is caught without a browser.
+"""
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+FRONTEND = Path(__file__).resolve().parents[2] / "frontend"
+APP_JS = FRONTEND / "assets" / "app.js"
+STYLES_CSS = FRONTEND / "assets" / "styles.css"
+
+
+def _js() -> str:
+    return APP_JS.read_text(encoding="utf-8")
+
+
+def _css() -> str:
+    return STYLES_CSS.read_text(encoding="utf-8")
+
+
+def _thinking_text_rule(css: str) -> str:
+    """Body of the `details.thinking .thinking-text { ... }` rule."""
+    rule = re.search(
+        r"details\.thinking \.thinking-text \{([\s\S]*?)\n\}", css
+    )
+    assert rule, "styles.css must style details.thinking .thinking-text"
+    return rule.group(1)
+
+
+def test_thinking_text_is_live_tail_clip() -> None:
+    """The window stays the fixed 320px clip (owner-confirmed: no
+    auto-height growth) but is NO LONGER user-scrollable."""
+    body = _thinking_text_rule(_css())
+    assert "max-height: 320px" in body, "the 320px clip must stay"
+    assert "overflow-y: hidden" in body, "the window must not scroll"
+    assert "overflow-y: auto" not in body, "no user-scrollable window remains"
+    assert "overflow-y: scroll" not in body
+
+
+def test_thinking_text_carries_owner_choice_comment() -> None:
+    """The owner-choice comment explains WHY the window is a live tail —
+    the phase-17 JS bottom-pin is the sole scroller."""
+    body = _thinking_text_rule(_css())
+    assert "owner choice 2026-08-24" in body
+    assert "live tail" in body
+    assert "sole scroller" in body
+
+
+def test_js_bottom_pin_intact_and_sole_scroller() -> None:
+    """The live-tail mechanism (phase 17) must survive phase 21 untouched:
+    the streaming `thinking` branch pins `textEl.scrollTop =
+    textEl.scrollHeight` per chunk, and it is the ONLY scrollTop
+    assignment in app.js (no new user-facing scroll code was added to the
+    window)."""
+    js = _js()
+    pin = "textEl.scrollTop = textEl.scrollHeight"
+    assert js.count(pin) == 1, "the bottom-pin must exist exactly once"
+    # It lives in the streaming thinking branch (before the delta branch),
+    # inside the `block.open` guard so closed blocks are not scrolled.
+    thinking_idx = js.find('ev.type === "thinking"')
+    delta_idx = js.find('ev.type === "delta"')
+    assert -1 < thinking_idx < delta_idx
+    thinking_branch = js[thinking_idx:delta_idx]
+    assert pin in thinking_branch
+    assert "if (block.open)" in thinking_branch
+
+
+def test_no_js_change_to_thinking_scroll_behavior() -> None:
+    """Phase 21 is CSS-only: nothing else in app.js touches the
+    .thinking-text scroll (no scroll-behavior, no wheel/touch handlers, no
+    scrollIntoView on the block — the page-level reveal stays the
+    phase-18 scrollReveal, which is not a .thinking-text scroller)."""
+    js = _js()
+    block_template = js.find('<div class="thinking-text">')
+    assert block_template != -1, "the thinking block template must exist"
+    # No inline scroll styling on the element itself.
+    assert 'style="scroll' not in js
+    assert "scroll-behavior" not in js
+    assert "addEventListener(\"wheel\"" not in js
+    assert "addEventListener('wheel'" not in js