fix(sync): deduplicate overlapping delta after coalesced part.updated (#916)

* fix: hide archived section and empty folders when no sessions remain

- Only push archived group in useSessionGrouping when there are archived
  sessions, preventing an empty archived section from rendering
- Hide empty folders in archived bucket via shouldKeepFolder check in
  SessionGroupSection (folders with no sessions and no content in
  children are filtered out)
- Always filter folders through shouldKeepFolder, not just during search

* perf: memoize archived folder filtering

* fix(sync): deduplicate overlapping delta after coalesced part.updated

When message.part.updated coalesces in the event pipeline and a
message.part.delta for the same part arrives in the same flush window,
the reducer appends the delta verbatim to the already-complete field
value, producing duplicated text in tool output and assistant messages.

Add targeted overlap reconciliation: only when a part.updated replaces
an existing part with overlapping string content, mark the next delta
for that field as dedupe-eligible. Normal streaming deltas remain
untouched (pure append).

Covers four cases:
- Full overlap: delta already present -> no-op
- Partial overlap: only non-overlapping suffix appended
- No overlap: unchanged append behavior
- Legitimate repeated output (ha + ha -> haha): preserved

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/sync/event-reducer.ts

@@ -16,6 +16,39 @@ import { stripSessionDiffSnapshots } from "./sanitize"
 import { syncDebug } from "./debug"
 
 const SKIP_PARTS = new Set(["patch", "step-start", "step-finish"])
+const DELTA_OVERLAP_FIELDS = ["text", "output"] as const
+
+type DedupeMetadata = {
+  __dedupeNextDeltaFields?: string[]
+}
+
+function appendNonOverlappingDelta(existingValue: string | undefined, delta: string) {
+  if (!existingValue || delta.length === 0) return (existingValue ?? "") + delta
+  if (existingValue.endsWith(delta)) return existingValue
+
+  const maxOverlap = Math.min(existingValue.length, delta.length)
+  for (let overlap = maxOverlap; overlap > 0; overlap--) {
+    if (existingValue.endsWith(delta.slice(0, overlap))) {
+      return existingValue + delta.slice(overlap)
+    }
+  }
+
+  return existingValue + delta
+}
+
+function getUpdatedDeltaFields(previous: Part, next: Part) {
+  const dedupeFields: string[] = []
+  for (const field of DELTA_OVERLAP_FIELDS) {
+    const previousValue = (previous as Record<string, unknown>)[field]
+    const nextValue = (next as Record<string, unknown>)[field]
+    if (typeof previousValue !== "string" || typeof nextValue !== "string") continue
+    if (previousValue.length === 0 || nextValue.length === 0) continue
+    if (nextValue === previousValue || nextValue.startsWith(previousValue) || previousValue.startsWith(nextValue)) {
+      dedupeFields.push(field)
+    }
+  }
+  return dedupeFields
+}
 
 // ---------------------------------------------------------------------------
 // Global events
@@ -205,7 +238,11 @@ export function applyDirectoryEvent(
       const next = [...parts]
       const result = Binary.search(next, part.id, (p) => p.id)
       if (result.found) {
-        next[result.index] = part
+        const previous = next[result.index]
+        const dedupeFields = getUpdatedDeltaFields(previous, part)
+        next[result.index] = dedupeFields.length > 0
+          ? { ...part, __dedupeNextDeltaFields: dedupeFields } as unknown as Part
+          : part
       } else {
         // Replace optimistic part (no sessionID) with server part of same type.
         // Gate: only scan if the first part lacks sessionID (optimistic parts are
@@ -262,9 +299,15 @@ export function applyDirectoryEvent(
       }
       const existing = parts[result.index] as Record<string, unknown>
       const existingValue = existing[props.field] as string | undefined
+      const dedupeFields = (existing as DedupeMetadata).__dedupeNextDeltaFields ?? []
+      const shouldDedupe = dedupeFields.includes(props.field)
       // Create new Part object + new array so React detects the change
       const next = [...parts]
-      next[result.index] = { ...existing, [props.field]: (existingValue ?? "") + props.delta } as Part
+      next[result.index] = {
+        ...existing,
+        [props.field]: shouldDedupe ? appendNonOverlappingDelta(existingValue, props.delta) : (existingValue ?? "") + props.delta,
+        __dedupeNextDeltaFields: dedupeFields.filter((field) => field !== props.field),
+      } as unknown as Part
       draft.part[props.messageID] = next
       return true
     }