feat(#4006): default auto-follow ON (Codex/Claude-Code parity) + fix default-hydration mismatch + CHANGELOG v0.51.378

Per maintainer decision, auto_scroll_follow defaults TRUE (sticky-bottom: follow
new output while streaming, scroll-up unpins and is respected) — matching Codex
CLI / Claude Code. The PR shipped it default-False.

Default-mismatch bug fixed (would have shown the True default as OFF for existing
users with no saved value):
- boot.js settings path: !!s.auto_scroll_follow -> s.auto_scroll_follow!==false
- boot.js no-settings fallback: false -> true
- panels.js checkbox render: !!settings.auto_scroll_follow -> !==false

Resolved the config.py + panels.js merge conflicts (kept both the Transparent
Stream setting and auto_scroll_follow). Added test_issue4006_auto_scroll_follow_default.py
pinning the True default across config/boot/panels + the follow-gate references.

CHANGELOG.md

@@ -3,6 +3,16 @@
 
 ## [Unreleased]
 
+## [v0.51.378] — 2026-06-13 — Release MQ (don't yank the viewport while reading; auto-follow toggle, #4006)
+
+### Fixed
+
+- **Streaming no longer snaps you to the bottom when you've scrolled up to read (#4006).** Two pin-hysteresis bugs are fixed: `_scrollAfterMessageRender` now honors `_messageUserUnpinned` (so a forced follow can't override a deliberate scroll-up after the 250px near-bottom counter re-pins), and `_finishDone`'s completion scroll is gated on an actual near-bottom check instead of firing unconditionally. (#4006)
+
+### Added
+
+- **Auto-follow toggle (Settings → Appearance).** A new "Auto-follow new content" setting controls whether the transcript follows new output to the bottom while streaming. It defaults **on** — a Codex/Claude-Code-style sticky bottom: you stay pinned to the latest output until you scroll up, at which point you're left where you are. Turn it off to always manage the scroll position yourself. (#4006)
+
 ## [v0.51.377] — 2026-06-13 — Release MP (Firefox post-stream scroll jitter, #3920)
 
 ### Fixed

tests/test_issue4006_auto_scroll_follow_default.py

@@ -0,0 +1,74 @@
+"""Pin the auto_scroll_follow setting's default + hydration consistency (#4006).
+
+The viewport-follow default is `True` (Codex/Claude-Code-style sticky bottom:
+follow new output to the bottom while streaming, but a deliberate scroll-up
+unpins and is respected). This pins the default in every place it is read so a
+future edit can't silently flip it or, worse, default it ON in config.py while
+hydrating it OFF in the browser (the classic default-mismatch bug, where an
+existing user with no saved value sees the feature as disabled).
+"""
+import pathlib
+import re
+
+ROOT = pathlib.Path(__file__).resolve().parent.parent
+
+
+def _read(rel):
+    return (ROOT / rel).read_text(encoding="utf-8")
+
+
+def test_auto_scroll_follow_default_is_true_in_config():
+    src = _read("api/config.py")
+    assert re.search(r'["\']auto_scroll_follow["\']\s*:\s*True', src), (
+        "auto_scroll_follow must default to True in _SETTINGS_DEFAULTS "
+        "(sticky-bottom follow; scroll-up unpins)"
+    )
+
+
+def test_auto_scroll_follow_in_bool_keys():
+    src = _read("api/config.py")
+    m = re.search(r"_SETTINGS_BOOL_KEYS\s*=\s*\{([^}]+)\}", src, re.DOTALL)
+    assert m, "_SETTINGS_BOOL_KEYS not found"
+    assert "auto_scroll_follow" in m.group(1), (
+        "auto_scroll_follow must be in _SETTINGS_BOOL_KEYS so it round-trips as a bool"
+    )
+
+
+def test_boot_hydration_defaults_true_when_setting_absent():
+    """boot.js must hydrate _autoScrollFollow as True when the saved settings
+    omit the key — `!!s.auto_scroll_follow` would wrongly default it OFF for
+    every existing user, contradicting the config.py default."""
+    src = _read("static/boot.js")
+    # Settings path: default-true read (=== false), not the truthy-coerce form.
+    assert "window._autoScrollFollow=s.auto_scroll_follow!==false" in src, (
+        "boot.js settings path must default _autoScrollFollow True when absent"
+    )
+    assert "window._autoScrollFollow=!!s.auto_scroll_follow" not in src, (
+        "boot.js must not use !!s.auto_scroll_follow — that defaults the True "
+        "setting OFF for users with no saved value"
+    )
+    # Fallback (no-settings) path must also default true.
+    assert "window._autoScrollFollow=true" in src, (
+        "boot.js fallback path must default _autoScrollFollow to true"
+    )
+
+
+def test_settings_checkbox_renders_checked_by_default():
+    """The Appearance checkbox must render checked when the setting is absent,
+    matching the True default (panels.js settings-load)."""
+    src = _read("static/panels.js")
+    assert "autoScrollFollowCb.checked=settings.auto_scroll_follow!==false" in src, (
+        "the auto-follow checkbox must default checked (=== false), not "
+        "!!settings.auto_scroll_follow which would render it unchecked by default"
+    )
+
+
+def test_follow_gate_references_auto_scroll_follow_and_unpin():
+    """The DOM-replace follow gate must consult both _autoScrollFollow (the
+    setting) and _messageUserUnpinned (the user's scroll-up), so the opt-out and
+    the read-while-streaming behaviors both hold."""
+    src = _read("static/ui.js")
+    assert "_shouldFollowMessagesOnDomReplace" in src
+    assert "_autoScrollFollow" in src and "_messageUserUnpinned" in src, (
+        "the follow gate must reference both _autoScrollFollow and _messageUserUnpinned"
+    )