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

packages/app/src/context/server-sdk.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test"
-import { resumeStreamAfterPageShow } from "./server-sdk"
+import { coalesceServerEvents, resumeStreamAfterPageShow } from "./server-sdk"
+import type { Event } from "@opencode-ai/sdk/v2/client"
 
 describe("resumeStreamAfterPageShow", () => {
   test("restarts a stream only after a back-forward cache restore", () => {
@@ -12,3 +13,41 @@ describe("resumeStreamAfterPageShow", () => {
     expect(starts).toBe(1)
   })
 })
+
+describe("coalesceServerEvents", () => {
+  const delta = (value: string, field = "text") => ({
+    directory: "/repo",
+    payload: {
+      type: "message.part.delta",
+      properties: { messageID: "msg", partID: "part", field, delta: value },
+    } as Event,
+  })
+
+  test("merges adjacent deltas for the same field", () => {
+    const result = coalesceServerEvents([delta("hello "), delta("world")])
+
+    expect(result).toHaveLength(1)
+    expect(result[0]?.payload).toMatchObject({ properties: { delta: "hello world" } })
+  })
+
+  test("preserves event boundaries and distinct fields", () => {
+    const status = {
+      directory: "/repo",
+      payload: { type: "session.status", properties: { sessionID: "ses", status: { type: "idle" } } } as Event,
+    }
+    const result = coalesceServerEvents([delta("a"), delta("b", "metadata"), status, delta("c")])
+
+    expect(result.map((event) => event.payload.type)).toEqual([
+      "message.part.delta",
+      "message.part.delta",
+      "session.status",
+      "message.part.delta",
+    ])
+  })
+
+  test("drops stale deltas", () => {
+    const result = coalesceServerEvents([delta("stale")], new Set(["/repo:msg:part"]))
+
+    expect(result).toEqual([])
+  })
+})