feat(app): make session timelines much faster AND without flicker or scroll jumps (#32331)

packages/app/src/context/global-sync/event-reducer.test.ts

@@ -134,6 +134,26 @@ describe("applyGlobalEvent", () => {
 })
 
 describe("applyDirectoryEvent", () => {
+  test("initializes text delta accumulation from the current part text", () => {
+    const part = { ...textPart("part", "session", "message"), text: "existing" }
+    const [store, setStore] = createStore(baseState({ part: { message: [part] } }))
+
+    applyDirectoryEvent({
+      event: {
+        type: "message.part.delta",
+        properties: { messageID: "message", partID: "part", field: "text", delta: " appended" },
+      },
+      store,
+      setStore,
+      push() {},
+      directory: "/tmp",
+      loadLsp() {},
+    })
+
+    expect(store.part_text_accum_delta.part).toBe("existing appended")
+    expect((store.part.message?.[0] as { text: string }).text).toBe("existing appended")
+  })
+
   test("preserves a Home-specific retained session limit", () => {
     const [store, setStore] = createStore(
       baseState({

packages/app/src/context/global-sync/event-reducer.ts

@@ -282,7 +282,13 @@ export function applyDirectoryEvent(input: {
       if (!parts) break
       const result = Binary.search(parts, props.partID, (p) => p.id)
       if (!result.found) break
-      input.setStore("part_text_accum_delta", props.partID, (existing) => (existing ?? "") + props.delta)
+      const field = props.field as keyof (typeof parts)[number]
+      const current = parts[result.index]?.[field]
+      input.setStore(
+        "part_text_accum_delta",
+        props.partID,
+        (existing) => (existing ?? (typeof current === "string" ? current : "")) + props.delta,
+      )
       input.setStore(
         "part",
         props.messageID,