fix: _loadOlderMessages scrolls to bottom instead of preserving position

When _loadOlderMessages prepends older messages, the viewport snaps
to the bottom instead of staying where the user was.

Two bugs compounding:
1. Wrong scrollable container. Code used `$("msgInner")` for scrollHeight
   and scrollTop, but #msgInner has no overflow-y — it is a flex column.
   The actual scrollable container is #messages (`.messages{overflow-y:auto}`).
   Setting msgInner.scrollTop was silently ignored.
2. renderMessages calls scrollToBottom at the end (ui.js:2552),
   which unconditionally scrolls #messages to the bottom and sets
   _scrollPinned=true. Since bug #1 made the scroll-restore a no-op,
   the page landed at the bottom every time.

Fix:
- Changed scroll restore target from `$("msgInner")` to `$("messages")`.
- Reset _scrollPinned = false after restoring the user position,
  so scrollToBottom does not re-fire on next tick.

static/sessions.js

@@ -385,17 +385,23 @@ async function _loadOlderMessages() {
     const olderMsgs = (data.session.messages || []).filter(m => m && m.role);
     if (!olderMsgs.length) { _messagesTruncated = false; return; }
     // Prepend older messages
-    const inner = $('msgInner');
-    const prevScrollH = inner ? inner.scrollHeight : 0;
+    // Use $('messages') — the scrollable container (#msgInner is not scrollable).
+    const container = $('messages');
+    const prevScrollH = container ? container.scrollHeight : 0;
     S.messages = [...olderMsgs, ...S.messages];
     _messagesTruncated = !!data.session._messages_truncated;
     _oldestIdx = data.session._messages_offset || 0;
     renderMessages();
-    // Restore scroll position so the user stays at the same message
-    if (inner) {
-      const newScrollH = inner.scrollHeight;
-      inner.scrollTop = newScrollH - prevScrollH;
+    // Restore scroll position so the user stays at the same message.
+    // renderMessages() calls scrollToBottom() at the end, so we must
+    // counter-scroll to where the user was before loading older messages.
+    if (container) {
+      const newScrollH = container.scrollHeight;
+      container.scrollTop = newScrollH - prevScrollH;
     }
+    // renderMessages() called scrollToBottom() which set _scrollPinned=true.
+    // We just restored the user's scroll position, so mark as not pinned.
+    _scrollPinned = false;
   } catch(e) {
     console.warn('_loadOlderMessages failed:', e);
   } finally {

tests/test_parallel_session_switch.py

@@ -557,3 +557,54 @@ class TestSessionSwitchCancellation:
         assert active_check_idx >= 0 and mutation_idx >= 0 and active_check_idx < mutation_idx, (
             "Active-session guard must run before S.messages mutation."
         )
+
+
+# ── 6. Scroll position preservation ──────────────────────────────────────────
+
+
+class TestScrollPositionPreservation:
+    """When _loadOlderMessages prepends messages, the user's scroll position
+    must be preserved — not snapped to the bottom.
+
+    The scrollable container is #messages (overflow-y:auto), not #msgInner
+    (which is a flex column with no overflow).  Also, renderMessages() calls
+    scrollToBottom() at the end, so _scrollPinned must be reset."""
+
+    def test_uses_correct_scrollable_container(self):
+        """_loadOlderMessages must use $('messages') not $('msgInner')."""
+        SESSIONS_JS = pathlib.Path(__file__).parent.parent / "static" / "sessions.js"
+        src = SESSIONS_JS.read_text(encoding="utf-8")
+
+        fn_start = src.find("async function _loadOlderMessages")
+        fn_end = src.find("\n}", fn_start) + 2
+        fn_body = src[fn_start:fn_end]
+
+        assert "$('messages')" in fn_body, (
+            "_loadOlderMessages should use $('messages') as the scrollable container "
+            "(#messages has overflow-y:auto). #msgInner has no overflow and is not scrollable."
+        )
+        assert "$('msgInner')" not in fn_body, (
+            "_loadOlderMessages must NOT use $('msgInner') for scroll position — "
+            "#msgInner is a flex column with no overflow-y."
+        )
+
+    def test_resets_scroll_pinned_after_restore(self):
+        """_scrollPinned must be set to false after restoring scroll position."""
+        SESSIONS_JS = pathlib.Path(__file__).parent.parent / "static" / "sessions.js"
+        src = SESSIONS_JS.read_text(encoding="utf-8")
+
+        fn_start = src.find("async function _loadOlderMessages")
+        fn_end = src.find("\n}", fn_start) + 2
+        fn_body = src[fn_start:fn_end]
+
+        assert "_scrollPinned = false" in fn_body, (
+            "renderMessages() calls scrollToBottom() which sets _scrollPinned=true. "
+            "After restoring the user's scroll position we must set _scrollPinned=false "
+            "to prevent the next render from snapping back to the bottom."
+        )
+        # _scrollPinned must appear after the scrollTop restore
+        restore_idx = fn_body.find("container.scrollTop = newScrollH - prevScrollH")
+        pinned_idx = fn_body.find("_scrollPinned = false")
+        assert restore_idx >= 0 and pinned_idx >= 0 and restore_idx < pinned_idx, (
+            "_scrollPinned = false must appear AFTER the scrollTop restore."
+        )