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

packages/ui/src/components/markdown-code-state.test.ts

@@ -0,0 +1,32 @@
+import { expect, test } from "bun:test"
+import { shouldResetCodeTokens } from "./markdown-code-state"
+
+const previous = {
+  language: "ts",
+  generation: 1,
+  stableCount: 3,
+  unstable: [],
+  raw: "```ts\nconst x = 1\n```",
+}
+
+test("resets tokens for a non-prefix replacement with the same generation and token count", () => {
+  expect(
+    shouldResetCodeTokens(previous, {
+      language: "ts",
+      generation: 1,
+      stableCount: 3,
+      raw: "```ts\nlet y = 2\n```",
+    }),
+  ).toBe(true)
+})
+
+test("retains tokens for an append-only streaming update", () => {
+  expect(
+    shouldResetCodeTokens(previous, {
+      language: "ts",
+      generation: 1,
+      stableCount: 4,
+      raw: `${previous.raw}\nmore`,
+    }),
+  ).toBe(false)
+})