fix(ui): keep partial markdown readable while responses stream (#19403)

packages/ui/src/components/markdown-stream.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from "bun:test"
+import { stream } from "./markdown-stream"
+
+describe("markdown stream", () => {
+  test("heals incomplete emphasis while streaming", () => {
+    expect(stream("hello **world", true)).toEqual([{ raw: "hello **world", src: "hello **world**", mode: "live" }])
+    expect(stream("say `code", true)).toEqual([{ raw: "say `code", src: "say `code`", mode: "live" }])
+  })
+
+  test("keeps incomplete links non-clickable until they finish", () => {
+    expect(stream("see [docs](https://example.com/gu", true)).toEqual([
+      { raw: "see [docs](https://example.com/gu", src: "see docs", mode: "live" },
+    ])
+  })
+
+  test("splits an unfinished trailing code fence from stable content", () => {
+    expect(stream("before\n\n```ts\nconst x = 1", true)).toEqual([
+      { raw: "before\n\n", src: "before\n\n", mode: "live" },
+      { raw: "```ts\nconst x = 1", src: "```ts\nconst x = 1", mode: "live" },
+    ])
+  })
+
+  test("keeps reference-style markdown as one block", () => {
+    expect(stream("[docs][1]\n\n[1]: https://example.com", true)).toEqual([
+      {
+        raw: "[docs][1]\n\n[1]: https://example.com",
+        src: "[docs][1]\n\n[1]: https://example.com",
+        mode: "live",
+      },
+    ])
+  })
+})