Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/app/src/context/global-sdk-event-queue.ts`:
- Around line 66-72: The loop that prunes stale deltas on message.part.updated
currently scans the entire result array (in global-sdk-event-queue.ts inside the
handler using partKey(event) and rebuildReplaceable), causing O(n²) behavior
under many updates; instead maintain an index (e.g., a Map keyed by partKey)
that maps each partKey to the queued delta entries (or their indices/IDs) so
that when handling a "message.part.updated" you only look up and remove matching
deltas from result via that map, and keep the map updated whenever you
push/splice entries and call rebuildReplaceable to preserve correctness.

---

Nitpick comments:
In `@packages/app/src/context/global-sdk-event-queue.test.ts`:
- Around line 37-73: Add a regression test that asserts directory isolation:
within the same describe block add a test that uses coalesceQueuedEvents with
queued events created via queued(delta(...), updated(...), status(...)) where
the deltas/updates share the same messageID/partID/field but have different
directory values; verify using eventTypes(events) and deltas(events) that events
from different directory values are never merged or pruned together (i.e., they
remain separate entries and stale-to-updated pruning only applies within the
same directory), referencing the existing helpers coalesceQueuedEvents, queued,
delta, updated, status, eventTypes and deltas to locate where to add and what to
assert.

In `@packages/app/src/pages/session/use-session-history-window.test.ts`:
- Around line 87-89: Rename the test whose body calls resolveHistoryTurnStart({
mode: "bottom", storedTurnStart: 15, length: 26, userScrolled: true }) so its
description reflects bottom-mode scroll-freeze behavior instead of "reading
mode" (e.g., change the test name string from "reading mode does not slide the
window when the user scrolls upward inside the rendered range" to something like
"bottom mode preserves storedTurnStart when user has scrolled inside rendered
range"); update only the test description text associated with that assertion to
match the actual scenario using resolveHistoryTurnStart and leave the assertion
logic unchanged.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/app/src/pages/session/use-session-history-window.test.ts`:
- Around line 16-40: The harness uses plain locals (messages, userScrolled) so
Solid reactivity in createSessionHistoryWindow never sees changes; update
createHarness to expose reactive sources (e.g., use createSignal or createStore)
for messages and userScrolled and pass their getter functions into
createSessionHistoryWindow (so visibleUserMessages and userScrolled read the
reactive getters), then implement setCount and setUserScrolled to call the
corresponding signal/setter to trigger memo/effect invalidation in the hook.

In `@packages/app/src/pages/session/use-session-history-window.ts`:
- Around line 276-284: The effect currently collapses every non-"bottom" mode
into returnToLatestIfFollowing(), which causes "hash" mode to be dropped
unintentionally; update the branches that check userScrolled and state.mode so
that returnToLatestIfFollowing() (and any implicit resumeLatestWindow behavior)
only runs when state.mode === "reading" (or when explicitly resuming), and do
not auto-exit when state.mode === "hash"; adjust the if (!userScrolled &&
state.mode !== "bottom") branch and the final fallback to guard on state.mode
=== "reading" instead of all non-"bottom" modes so "hash" remains sticky until
an explicit clear/resume action.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/app/src/context/global-sdk-event-queue.ts`:
- Around line 5-26: The composite key helpers deltaKey, partKey and
replaceableKey currently join components with ":" which can collide if any
component (especially event.directory) contains ":"; update these functions to
produce unambiguous keys by encoding or serializing components (e.g., return
JSON.stringify([event.directory, props.messageID, props.partID, props.field])
for deltaKey, JSON.stringify([event.directory, props.messageID, props.partID])
for partKey, and JSON.stringify(["session.status", event.directory,
event.payload.properties.sessionID]) or JSON.stringify(["lsp.updated",
event.directory]) for replaceableKey) so keys are deterministic and
collision-free for all QueuedGlobalEvent payload shapes. Ensure all branches
return a string (not undefined) and keep the same semantics for different
payload types.

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@packages/app/src/pages/session/use-session-history-window.ts`:
- Around line 159-183: The async continuation after awaiting input.loadMore(...)
must abort if the UI mode or session changed while the request was in flight:
add guards right after each await (the one in this function and the same spot in
loadAndReveal) to return early unless input.sessionID() === id and the UI is
still in reading-mode (i.e. turnStart() === 0 or whatever current-mode predicate
you use); only then compute growth and call expandForReading(...). Ensure you
also preserve the existing sessionID check and the state.prefetchNoGrowth update
but do the mode/session checks before calling expandForReading.