feat(markdown): checkpoint streaming render — settled-block cache, highlight on close

2 files changed, 124 insertions, 7 deletions

diff --git a/src/features/markdown/ui/Markdown.svelte b/src/features/markdown/ui/Markdown.svelte
index 72b892b..da2c9ba 100644
--- a/src/features/markdown/ui/Markdown.svelte
+++ b/src/features/markdown/ui/Markdown.svelte
@@ -1,5 +1,6 @@
 <script lang="ts">
   import { renderMarkdown } from "../logic/markdown";
+  import { advance, createStreamingMarkdown } from "../logic/streaming-markdown";
 
   let {
     text,
@@ -10,15 +11,27 @@
     streaming?: boolean;
   } = $props();
 
-  // Pure transform; the HTML is already DOMPurify-sanitized in renderMarkdown.
-  const html = $derived(renderMarkdown(text, { streaming }));
+  // Checkpoint streaming state: one per component instance (per streaming
+  // message). It caches the settled blocks' HTML so each delta re-renders only
+  // the live tail — never the settled prefix (see logic/streaming-markdown).
+  const streamingState = createStreamingMarkdown();
+
+  // While streaming: the checkpoint split (settled prefix + live tail), which
+  // equals today's full render at every step modulo inter-block newlines and
+  // highlight-on-close. When the message completes (or for every non-streaming
+  // chunk): ONE full authoritative render — today's exact behavior, so the
+  // final/persisted output is byte-identical by construction.
+  const rendered = $derived.by(() => {
+    if (!streaming) return { prefixHtml: "", tailHtml: renderMarkdown(text) };
+    return advance(streamingState, text);
+  });
 
   let container: HTMLElement;
 
   // One delegated listener on the stable container handles every code block's
-  // copy button — including blocks re-created when `html` changes (streaming),
-  // since the listener lives on the container, not the buttons. Clipboard is the
-  // edge effect; absent (insecure context) → no-op.
+  // copy button — including blocks re-created when the tail html changes
+  // (streaming), since the listener lives on the container, not the buttons.
+  // Clipboard is the edge effect; absent (insecure context) → no-op.
   $effect(() => {
     const el = container;
     if (el === undefined) return;
@@ -53,6 +66,17 @@
 </script>
 
 <div class="markdown-body" bind:this={container}>
-  <!-- {@html} is safe here: `html` is DOMPurify-sanitized inside renderMarkdown. -->
-  {@html html}
+  {#if streaming}
+    <!-- Two sibling wrappers (no padding/border/min-height, so block margins
+         collapse across them exactly as if the blocks were direct children).
+         Svelte 5 skips the {@html} DOM write when the string is unchanged, so
+         the settled prefix's subtree is never re-parsed/re-created/re-laid-out;
+         only the tail's subtree is replaced per delta. Both strings are
+         DOMPurify-sanitized in the logic modules. -->
+    <div class="markdown-stream-prefix">{@html rendered.prefixHtml}</div>
+    <div class="markdown-stream-tail">{@html rendered.tailHtml}</div>
+  {:else}
+    <!-- {@html} is safe here: `html` is DOMPurify-sanitized inside renderMarkdown. -->
+    {@html rendered.tailHtml}
+  {/if}
 </div>
diff --git a/src/features/markdown/ui/markdown.test.ts b/src/features/markdown/ui/markdown.test.ts
index d65b3d1..721dddb 100644
--- a/src/features/markdown/ui/markdown.test.ts
+++ b/src/features/markdown/ui/markdown.test.ts
@@ -1,4 +1,5 @@
 import { fireEvent, render, screen } from "@testing-library/svelte";
+import hljs from "highlight.js/lib/core";
 import { describe, expect, it, vi } from "vitest";
 import Markdown from "./Markdown.svelte";
 
@@ -37,4 +38,96 @@ describe("Markdown", () => {
     expect(writeText).toHaveBeenCalledTimes(1);
     expect(writeText.mock.calls[0]?.[0]).toContain("const x = 1;");
   });
+
+  it("non-streaming renders a single container (today's full-render path)", () => {
+    const { container } = render(Markdown, { props: { text: "# Hello" } });
+    expect(container.querySelector(".markdown-stream-prefix")).toBeNull();
+    expect(container.querySelector(".markdown-stream-tail")).toBeNull();
+    // The rendered blocks are DIRECT children of .markdown-body (no wrappers).
+    const body = container.querySelector(".markdown-body");
+    expect(body?.querySelector("h1")).not.toBeNull();
+    expect(body?.children[0]?.tagName).toBe("H1");
+  });
+
+  it("streaming renders the prefix/tail split with content matching the full render", () => {
+    const { container } = render(Markdown, {
+      props: { text: "# Hello\n\nsettled paragraph\n\ngrowing tail", streaming: true },
+    });
+
+    const prefix = container.querySelector<HTMLElement>(".markdown-stream-prefix");
+    const tail = container.querySelector<HTMLElement>(".markdown-stream-tail");
+    expect(prefix).not.toBeNull();
+    expect(tail).not.toBeNull();
+    // Same visible content as today's render, split across the two containers.
+    expect(screen.getByRole("heading", { level: 1, name: "Hello" })).toBeInTheDocument();
+    expect(container.textContent).toContain("growing tail");
+    expect(prefix?.querySelector("h1")).not.toBeNull();
+  });
+
+  it("streaming: the settled prefix's html string identity is stable across text appends (no DOM churn)", async () => {
+    const spy = vi.spyOn(hljs, "highlight");
+    const { container, rerender } = render(Markdown, {
+      props: { text: "intro\n\n```js\nconst x = 1;\n```\n\nafter", streaming: true },
+    });
+
+    const prefixBefore = container.querySelector(".markdown-stream-prefix")?.innerHTML;
+    expect(prefixBefore).toContain("hljs-keyword"); // the closed fence settled, highlighted once
+    expect(spy).toHaveBeenCalledTimes(1);
+
+    // Appends that settle nothing leave the prefix string (and its DOM) alone.
+    await rerender({ text: "intro\n\n```js\nconst x = 1;\n```\n\nafter and", streaming: true });
+    await rerender({
+      text: "intro\n\n```js\nconst x = 1;\n```\n\nafter and more",
+      streaming: true,
+    });
+    expect(container.querySelector(".markdown-stream-prefix")?.innerHTML).toBe(prefixBefore);
+    expect(spy).toHaveBeenCalledTimes(1); // still exactly one highlight
+  });
+
+  it("streaming: an open fence renders uncolored, then highlights exactly once when it closes", async () => {
+    const spy = vi.spyOn(hljs, "highlight");
+    const { container, rerender } = render(Markdown, {
+      props: { text: "intro\n\n```js\nconst x", streaming: true },
+    });
+
+    const tail = container.querySelector<HTMLElement>(".markdown-stream-tail");
+    expect(tail?.innerHTML).toContain("<pre><code");
+    expect(tail?.innerHTML).toContain("data-copy"); // same copy button
+    expect(tail?.innerHTML).not.toContain("hljs-keyword"); // uncolored while open
+    expect(spy).not.toHaveBeenCalled();
+
+    // Close the fence → highlighted EXACTLY once, settled into the prefix.
+    await rerender({ text: "intro\n\n```js\nconst x = 1;\n```\n\ndone", streaming: true });
+    expect(spy).toHaveBeenCalledTimes(1);
+    expect(container.querySelector(".markdown-stream-prefix")?.innerHTML).toContain("hljs-keyword");
+  });
+
+  it("streaming → done flips to the single-container full render (completion backstop)", async () => {
+    const { container, rerender } = render(Markdown, {
+      props: { text: "# Hello\n\nbody", streaming: true },
+    });
+    expect(container.querySelector(".markdown-stream-prefix")).not.toBeNull();
+
+    await rerender({ text: "# Hello\n\nbody", streaming: false });
+    expect(container.querySelector(".markdown-stream-prefix")).toBeNull();
+    expect(container.querySelector(".markdown-stream-tail")).toBeNull();
+    expect(screen.getByRole("heading", { level: 1, name: "Hello" })).toBeInTheDocument();
+  });
+
+  it("streaming: the delegated copy-button listener still works for tail code blocks", async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    Object.defineProperty(navigator, "clipboard", { value: { writeText }, configurable: true });
+
+    const { container } = render(Markdown, {
+      props: { text: "para\n\n```js\nconst y = 2;", streaming: true },
+    });
+
+    const button = container.querySelector<HTMLElement>("[data-copy]");
+    expect(button).not.toBeNull();
+    if (button === null) throw new Error("expected a copy button");
+    await fireEvent.click(button);
+
+    expect(writeText).toHaveBeenCalledTimes(1);
+    expect(writeText.mock.calls[0]?.[0]).toContain("const y = 2;");
+  });
 });