perf(ui): reuse warm markdown blocks

packages/ui/src/components/chat/MarkdownRendererImpl.test.ts

@@ -1,9 +1,366 @@
-import { describe, expect, test } from 'bun:test';
+import { describe, expect, mock, test } from 'bun:test';
 
 import { localPathFromFileUrl, parseFileReference, type ParsedFileReference } from './fileReferenceParser';
 
 const parse = (value: string): ParsedFileReference | null => parseFileReference(value);
 
+type FakeElement = {
+    childNodes: FakeElement[];
+    children: FakeElement[];
+    parentNode: FakeElement | null;
+    attributes: Map<string, string>;
+    style: { display: string; setProperty: () => void };
+    innerHTML: string;
+    setAttribute: (name: string, value: string) => void;
+    getAttribute: (name: string) => string | null;
+    appendChild: (child: FakeElement) => FakeElement;
+    replaceWith: (replacement: FakeElement) => void;
+    remove: () => void;
+    querySelector: (selector: string) => FakeElement | null;
+    querySelectorAll: <T>(selector: string) => T[];
+    addEventListener: () => void;
+    removeEventListener: () => void;
+    contains: (child: FakeElement) => boolean;
+    isEqualNode: () => boolean;
+};
+
+type FakeDocument = { createElement: () => FakeElement };
+type FakeJsxProps = {
+    ref?: { current: FakeElement | null };
+    children?: FakeElement | FakeElement[];
+    className?: string;
+    'data-markdown-content'?: boolean;
+};
+
+let syncRenderCalls = 0;
+let morphCalls = 0;
+let decorateCalls = 0;
+let mermaidRegistryCreates = 0;
+let mermaidRegistryCleanups = 0;
+let cachedRendererBlocks: Array<{ id: string; html: string }> | null = null;
+let renderedRendererBlocks: Array<{ id: string; html: string }> = [];
+let currentContextVersion = 0;
+const layoutEffects: Array<() => void> = [];
+const passiveEffects: Array<() => void | (() => void)> = [];
+let hookCursor = 0;
+let hookStates: Array<{ current: null } | undefined> = [];
+let activeFakeDocument: FakeDocument | null = null;
+
+const makeFakeElement = (ownerDocument: { createElement: () => FakeElement }): FakeElement => {
+    let html = '';
+    const element: FakeElement = {
+        childNodes: [],
+        children: [],
+        parentNode: null,
+        attributes: new Map(),
+        style: { display: '', setProperty: () => undefined },
+        get innerHTML() {
+            return html;
+        },
+        set innerHTML(value: string) {
+            html = value;
+        },
+        setAttribute(name, value) {
+            this.attributes.set(name, value);
+        },
+        getAttribute(name) {
+            return this.attributes.get(name) ?? null;
+        },
+        appendChild(child) {
+            child.parentNode = this;
+            this.childNodes.push(child);
+            this.children.push(child);
+            return child;
+        },
+        replaceWith(replacement) {
+            if (!this.parentNode) return;
+            const parent = this.parentNode;
+            const index = parent.children.indexOf(this);
+            if (index < 0) return;
+            replacement.parentNode = parent;
+            parent.children[index] = replacement;
+            parent.childNodes[index] = replacement;
+            this.parentNode = null;
+        },
+        remove() {
+            if (!this.parentNode) return;
+            const parent = this.parentNode;
+            parent.children = parent.children.filter((child) => child !== this);
+            parent.childNodes = parent.childNodes.filter((child) => child !== this);
+            this.parentNode = null;
+        },
+        querySelector(selector) {
+            if (selector === '[data-markdown-content]') {
+                return this.children.find((child) => child.getAttribute('data-markdown-content') === '') ?? null;
+            }
+            if (selector === '[data-markdown="mermaid-block"]' && html.includes('data-markdown="mermaid-block"')) {
+                return this;
+            }
+            for (const child of this.children) {
+                const match = child.querySelector(selector);
+                if (match) return match;
+            }
+            return null;
+        },
+        querySelectorAll: () => [],
+        addEventListener: () => undefined,
+        removeEventListener: () => undefined,
+        contains(child) {
+            return child === this || this.children.some((candidate) => candidate.contains(child));
+        },
+        isEqualNode: () => false,
+    };
+    return element;
+};
+
+const installRendererDom = () => {
+    const previousDocument = Object.getOwnPropertyDescriptor(globalThis, 'document');
+    const previousWindow = Object.getOwnPropertyDescriptor(globalThis, 'window');
+    const previousMutationObserver = Object.getOwnPropertyDescriptor(globalThis, 'MutationObserver');
+    const documentStub: FakeDocument = { createElement: () => makeFakeElement(documentStub) };
+    activeFakeDocument = documentStub;
+    Object.defineProperty(globalThis, 'document', { configurable: true, value: documentStub });
+    Object.defineProperty(globalThis, 'window', {
+        configurable: true,
+        value: {
+            matchMedia: () => ({ matches: false }),
+            setTimeout,
+            clearTimeout,
+            requestAnimationFrame: (callback: FrameRequestCallback) => setTimeout(() => callback(Date.now()), 0),
+        },
+    });
+    Object.defineProperty(globalThis, 'MutationObserver', {
+        configurable: true,
+        value: class {
+            observe() {}
+            disconnect() {}
+        },
+    });
+    return () => {
+        if (previousDocument) Object.defineProperty(globalThis, 'document', previousDocument);
+        else Reflect.deleteProperty(globalThis, 'document');
+        if (previousWindow) Object.defineProperty(globalThis, 'window', previousWindow);
+        else Reflect.deleteProperty(globalThis, 'window');
+        if (previousMutationObserver) Object.defineProperty(globalThis, 'MutationObserver', previousMutationObserver);
+        else Reflect.deleteProperty(globalThis, 'MutationObserver');
+        activeFakeDocument = null;
+    };
+};
+
+const rendererThemes = [{
+    metadata: { id: 'renderer-test' },
+    colors: {
+        surface: { elevated: '#fff', foreground: '#000', mutedForeground: '#666', muted: '#eee' },
+        interactive: { border: '#ccc' },
+        primary: { base: '#00f' },
+    },
+}, {
+    metadata: { id: 'renderer-test-next' },
+    colors: {
+        surface: { elevated: '#eee', foreground: '#111', mutedForeground: '#555', muted: '#ddd' },
+        interactive: { border: '#bbb' },
+        primary: { base: '#f00' },
+    },
+}];
+let rendererThemeIndex = 0;
+const rendererTheme = () => rendererThemes[rendererThemeIndex] ?? rendererThemes[0];
+const rendererUiState = {
+    codeBlockLineWrap: false,
+    mermaidRenderingMode: 'svg',
+    setCodeBlockLineWrap: () => undefined,
+    openContextPreview: () => undefined,
+};
+
+const fakeReact = {
+    useCallback: <T>(callback: T): T => {
+        hookCursor += 1;
+        return callback;
+    },
+    useEffect: (effect: () => void | (() => void)) => { passiveEffects.push(effect); },
+    useLayoutEffect: (effect: () => void) => { layoutEffects.push(effect); },
+    useMemo: <T>(factory: () => T): T => {
+        hookCursor += 1;
+        return factory();
+    },
+    useRef: <T>(current: T) => {
+        const index = hookCursor;
+        hookCursor += 1;
+        if (!hookStates[index]) hookStates[index] = { current: null };
+        // SAFETY: this test hook preserves one mutable ref slot per hook index.
+        return hookStates[index] as { current: T };
+    },
+    memo: <T>(component: T): T => component,
+};
+
+const fakeJsx = (_type: string, props: FakeJsxProps | null, ...children: FakeElement[]): FakeElement => {
+    const ref = props?.ref;
+    // SAFETY: the renderer test installs the typed fake document before JSX is
+    // evaluated; this branch only supplies its fake element factory.
+    const fakeDocument = activeFakeDocument;
+    if (!fakeDocument) throw new Error('Renderer fake document is not installed');
+    const element = ref?.current ?? makeFakeElement(fakeDocument);
+    if (!ref?.current) {
+        element.childNodes.length = 0;
+        element.children.length = 0;
+    }
+    if (props) {
+        if (ref) ref.current = element;
+        if (props.className) element.setAttribute('class', props.className);
+        if (props['data-markdown-content']) element.setAttribute('data-markdown-content', '');
+    }
+    const jsxChildren = props?.children;
+    const allChildren = jsxChildren === undefined ? children : Array.isArray(jsxChildren) ? jsxChildren : [jsxChildren];
+    for (const child of allChildren) {
+        if (child) element.appendChild(child);
+    }
+    return element;
+};
+
+mock.module('react', () => ({ default: fakeReact }));
+mock.module('react/jsx-runtime', () => ({ jsx: fakeJsx, jsxs: fakeJsx, Fragment: 'fragment' }));
+mock.module('react/jsx-dev-runtime', () => ({ jsxDEV: fakeJsx, Fragment: 'fragment' }));
+mock.module('beautiful-mermaid', () => ({
+    renderMermaidASCII: () => '',
+    renderMermaidSVG: (_source: string, colors: { bg: string }) => colors.bg,
+}));
+mock.module('@/lib/utils', () => ({ cn: (...values: string[]) => values.filter(Boolean).join(' ') }));
+mock.module('@/lib/i18n', () => ({ useI18n: () => ({ t: (key: string) => `${key}:${currentContextVersion}` }) }));
+mock.module('@/lib/runtime-fetch', () => ({ runtimeFetch: async () => ({ ok: false }) }));
+mock.module('@/lib/url', () => ({
+    getUrlScheme: () => null,
+    isAppLinkUrl: () => false,
+    isExternalHttpUrl: () => false,
+    openConfirmedAppLinkUrl: async () => false,
+    openExternalUrl: async () => undefined,
+}));
+mock.module('@/contexts/useThemeSystem', () => ({ useOptionalThemeSystem: () => ({ currentTheme: rendererTheme() }) }));
+mock.module('@/lib/theme/themes', () => ({ getDefaultTheme: () => rendererTheme() }));
+mock.module('./message/FadeInOnReveal', () => ({ FadeInOnReveal: ({ children }: { children: FakeElement | FakeElement[] }) => children }));
+type RendererUiSelectorResult = boolean | string | (() => void);
+const fakeUseUIStore = Object.assign(
+    (selector: (state: typeof rendererUiState) => RendererUiSelectorResult) => selector(rendererUiState),
+    { getState: () => rendererUiState },
+);
+mock.module('@/stores/useUIStore', () => ({ useUIStore: fakeUseUIStore }));
+mock.module('@/hooks/useEffectiveDirectory', () => ({ useEffectiveDirectory: () => null }));
+mock.module('@/hooks/useRuntimeAPIs', () => ({ useRuntimeAPIs: () => ({ editor: undefined, runtime: { isVSCode: false } }) }));
+mock.module('@/lib/desktop', () => ({ isDesktopLocalOriginActive: () => false, isDesktopShell: () => false, isVSCodeRuntime: () => false }));
+mock.module('@/lib/runtimeSurface', () => ({ isMobileSurfaceRuntime: () => false }));
+mock.module('@/lib/outsideFileGrants', () => ({ ensureOutsideFileGrantForDesktop: async () => undefined }));
+mock.module('@/lib/path-utils', () => ({ getDirectoryForFilePath: () => '', isFilePathWithinDirectory: () => true, toAbsoluteFilePath: () => '' }));
+mock.module('./markdown/markdownCore', () => ({
+    getCachedMarkdownBlocks: () => cachedRendererBlocks,
+    renderMarkdownBlocks: async () => renderedRendererBlocks,
+    renderMarkdownSync: () => {
+        syncRenderCalls += 1;
+        return '<p>cold</p>';
+    },
+}));
+mock.module('./markdown/markdownTheme', () => ({ ensureMarkdownShikiTheme: () => undefined }));
+mock.module('./markdown/markdownSyntaxVars', () => ({ getMarkdownSyntaxVars: () => ({}) }));
+type TestDecorateContext = {
+    labels: { copy: string };
+    codeBlockLineWrap: boolean;
+    renderMermaid: (source: string) => { svg?: string };
+};
+mock.module('./markdown/decorate', () => ({
+    attachMarkdownInteractions: () => () => undefined,
+    applyMarkdownCodeBlockWrapState: () => undefined,
+    decorateMarkdown: (root: FakeElement, ctx: TestDecorateContext) => {
+        decorateCalls += 1;
+        if (root.getAttribute('data-test-decoration-marker') === 'true') return;
+        root.setAttribute('data-test-decoration-marker', 'true');
+        root.setAttribute(
+            'data-test-decoration',
+            `${ctx.labels.copy}|${ctx.codeBlockLineWrap}|${ctx.renderMermaid('test').svg ?? ''}`,
+        );
+    },
+    getMarkdownCodeText: () => '',
+}));
+mock.module('./markdown/textPosition', () => ({ findTextPosition: () => null }));
+mock.module('./markdown/mermaidViewer', () => ({
+    createMermaidViewerRegistry: () => {
+        mermaidRegistryCreates += 1;
+        return {
+            refresh: () => undefined,
+            cleanup: () => { mermaidRegistryCleanups += 1; },
+        };
+    },
+    MERMAID_BLOCK_SELECTOR: '[data-markdown="mermaid-block"]',
+    shouldRefreshMermaidViewers: (container: Pick<FakeElement, 'querySelector'>) => container.querySelector('[data-markdown="mermaid-block"]') !== null,
+}));
+mock.module('@/stores/utils/streamDebug', () => ({ streamPerfCount: () => undefined, streamPerfObserve: () => undefined }));
+mock.module('morphdom', () => ({ default: () => { morphCalls += 1; } }));
+
+const { MarkdownRenderer } = await import('./MarkdownRendererImpl');
+
+const resetRendererTestState = () => {
+    cachedRendererBlocks = null;
+    renderedRendererBlocks = [];
+    syncRenderCalls = 0;
+    morphCalls = 0;
+    decorateCalls = 0;
+    mermaidRegistryCreates = 0;
+    mermaidRegistryCleanups = 0;
+    hookCursor = 0;
+    hookStates = [];
+    layoutEffects.length = 0;
+    passiveEffects.length = 0;
+    currentContextVersion = 0;
+    rendererThemeIndex = 0;
+    rendererUiState.codeBlockLineWrap = false;
+};
+
+const beginRendererRender = () => {
+    hookCursor = 0;
+    return renderMarkdownForTest();
+};
+
+const rendererRoot = (value: ReturnType<typeof renderMarkdownForTest>): FakeElement => {
+    if (!(value instanceof Object) || !('childNodes' in value) || !('getAttribute' in value)) {
+        throw new Error('Renderer test did not return its fake JSX root');
+    }
+    // SAFETY: the structural check confirms this ReactNode is the object
+    // returned by the mocked JSX runtime.
+    const candidate = value as object;
+    // SAFETY: the mocked JSX runtime creates the complete FakeElement shape.
+    return candidate as FakeElement;
+};
+
+const runRendererLayoutEffects = () => {
+    const pending = layoutEffects.splice(0);
+    for (const effect of pending) effect();
+};
+
+const runRendererPassiveEffects = () => passiveEffects.splice(0).map((effect) => effect());
+
+const findBlock = (root: FakeElement, id: string): FakeElement | null => {
+    if (root.getAttribute('data-md-id') === id) return root;
+    for (const child of root.children) {
+        const match = findBlock(child, id);
+        if (match) return match;
+    }
+    return null;
+};
+
+const renderMarkdownForTest = () => MarkdownRenderer({
+    content: 'cached markdown',
+    messageId: 'message-1',
+    isAnimated: false,
+    isStreaming: false,
+});
+
+const withRendererDom = async (run: () => void | Promise<void>): Promise<void> => {
+    const restoreDom = installRendererDom();
+    const previousThemeIndex = rendererThemeIndex;
+    try {
+        await run();
+    } finally {
+        rendererThemeIndex = previousThemeIndex;
+        restoreDom();
+    }
+};
+
 describe('parseFileReference', () => {
     test('returns null for empty or whitespace input', () => {
         expect(parse('')).toBeNull();
@@ -72,11 +429,7 @@ describe('parseFileReference', () => {
     });
 
     test('preserves line:col form (does not interpret as range)', () => {
-        expect(parse('src/foo.ts:42:8')).toEqual({
+        expect(parse('src/foo.ts:42:8')).toEqual({ path: 'src/foo.ts', line: 42, column: 8 });
-            path: 'src/foo.ts',
-            line: 42,
-            column: 8,
-        });
     });
 
     test('preserves hash form', () => {
@@ -110,3 +463,95 @@ describe('localPathFromFileUrl', () => {
         expect(localPathFromFileUrl('file:///tmp/bad%ZZpath')).toBeNull();
     });
 });
+
+describe('MarkdownRenderer warm settled path', () => {
+    test('installs cached blocks without sync fallback and skips same-ID morph', async () => {
+        await withRendererDom(async () => {
+            resetRendererTestState();
+            cachedRendererBlocks = [{ id: 'full:cached', html: '<p>cached</p>' }];
+            renderedRendererBlocks = cachedRendererBlocks;
+            syncRenderCalls = 0;
+            morphCalls = 0;
+            decorateCalls = 0;
+
+            // SAFETY: the test JSX adapter returns the fake element assigned to the
+            // renderer container ref and exposes the DOM members used below.
+            const root = rendererRoot(beginRendererRender());
+            runRendererLayoutEffects();
+            expect(syncRenderCalls).toBe(0);
+            const block = findBlock(root, 'full:cached');
+            expect(block).not.toBeNull();
+            expect(block?.innerHTML).toBe('<p>cached</p>');
+            expect(block?.getAttribute('data-md-block')).toBe('');
+            expect(block?.getAttribute('data-md-id')).toBe('full:cached');
+            expect(block?.style.display).toBe('contents');
+            expect(decorateCalls).toBe(1);
+
+            runRendererPassiveEffects();
+            await Promise.resolve();
+            expect(morphCalls).toBe(0);
+        });
+    });
+
+    test('recreates the Mermaid registry after StrictMode-like cleanup without remounting blocks', () => {
+        return withRendererDom(() => {
+            resetRendererTestState();
+            const mermaidHtml = '<div data-markdown="mermaid-block"><svg></svg></div>';
+            cachedRendererBlocks = [{ id: 'full:mermaid', html: mermaidHtml }];
+            renderedRendererBlocks = cachedRendererBlocks;
+            mermaidRegistryCreates = 0;
+            mermaidRegistryCleanups = 0;
+            morphCalls = 0;
+
+            const root = rendererRoot(beginRendererRender());
+            runRendererLayoutEffects();
+            expect(mermaidRegistryCreates).toBe(1);
+            const cleanups = runRendererPassiveEffects();
+            for (const cleanup of cleanups) cleanup?.();
+            expect(mermaidRegistryCleanups).toBe(1);
+
+            beginRendererRender();
+            runRendererLayoutEffects();
+            expect(mermaidRegistryCreates).toBe(2);
+            expect(findBlock(root, 'full:mermaid')).not.toBeNull();
+            expect(morphCalls).toBe(0);
+        });
+    });
+
+    test('redecorates a same-ID block when decoration context changes before async completion', async () => {
+        await withRendererDom(async () => {
+            resetRendererTestState();
+            cachedRendererBlocks = [{
+                id: 'full:context',
+                html: '<div data-markdown="mermaid-block"><p>cached</p></div>',
+            }];
+            renderedRendererBlocks = cachedRendererBlocks;
+
+            const root = rendererRoot(beginRendererRender());
+            runRendererLayoutEffects();
+            const block = findBlock(root, 'full:context');
+            const firstDecorationId = block?.getAttribute('data-md-decoration-id');
+            expect(firstDecorationId).not.toBeNull();
+            const firstDecorateCalls = decorateCalls;
+
+            rendererThemeIndex = 1;
+            currentContextVersion = 1;
+            rendererUiState.codeBlockLineWrap = true;
+            beginRendererRender();
+            runRendererLayoutEffects();
+            runRendererPassiveEffects();
+            await Promise.resolve();
+
+            expect(decorateCalls).toBeGreaterThan(firstDecorateCalls);
+            expect(syncRenderCalls).toBe(0);
+            expect(morphCalls).toBe(0);
+            const updatedBlock = findBlock(root, 'full:context');
+            expect(updatedBlock?.getAttribute('data-md-decoration-id')).not.toBe(firstDecorationId);
+            expect(updatedBlock?.getAttribute('data-test-decoration')).toContain(':1|true|#eee');
+            expect(updatedBlock?.getAttribute('data-test-decoration-marker')).toBe('true');
+            expect(mermaidRegistryCleanups).toBeGreaterThan(0);
+            expect(mermaidRegistryCreates).toBeGreaterThan(1);
+        });
+    });
+
+});

packages/ui/src/components/chat/MarkdownRendererImpl.tsx

@@ -20,7 +20,12 @@ import { isDesktopLocalOriginActive, isDesktopShell, isVSCodeRuntime } from '@/l
 import { isMobileSurfaceRuntime } from '@/lib/runtimeSurface';
 import { ensureOutsideFileGrantForDesktop } from '@/lib/outsideFileGrants';
 import { getDirectoryForFilePath, isFilePathWithinDirectory, toAbsoluteFilePath } from '@/lib/path-utils';
-import { renderMarkdownBlocks, renderMarkdownSync, type MarkdownImageMode } from './markdown/markdownCore';
+import {
+  getCachedMarkdownBlocks,
+  renderMarkdownBlocks,
+  renderMarkdownSync,
+  type MarkdownImageMode,
+} from './markdown/markdownCore';
 import { ensureMarkdownShikiTheme } from './markdown/markdownTheme';
 import { getMarkdownSyntaxVars } from './markdown/markdownSyntaxVars';
 import {
@@ -659,6 +664,18 @@ const useMermaidInlineInteractions = ({
 // so a stable diagram is laid out once and served from cache thereafter.
 const MERMAID_RENDER_CACHE = new Map<string, MermaidRender>();
 const MERMAID_RENDER_CACHE_MAX = 100;
+const MARKDOWN_DECORATION_ID_ATTR = 'data-md-decoration-id';
+const MARKDOWN_DECORATION_IDS = new WeakMap<DecorateContext, string>();
+let nextMarkdownDecorationId = 0;
+
+const getMarkdownDecorationId = (ctx: DecorateContext): string => {
+  const existing = MARKDOWN_DECORATION_IDS.get(ctx);
+  if (existing) return existing;
+  const id = `decoration-${nextMarkdownDecorationId}`;
+  nextMarkdownDecorationId += 1;
+  MARKDOWN_DECORATION_IDS.set(ctx, id);
+  return id;
+};
 
 const cachedMermaidRender = (key: string, compute: () => MermaidRender): MermaidRender => {
   const existing = MERMAID_RENDER_CACHE.get(key);
@@ -780,25 +797,40 @@ const useMorphdomMarkdown = ({
     const container = containerRef.current;
     const target = container?.querySelector<HTMLElement>('[data-markdown-content]') ?? container;
     if (!target) return;
+    const decorationId = getMarkdownDecorationId(ctx);
     if (text && target.childNodes.length === 0) {
-      const block = document.createElement('div');
+      const cachedBlocks = !streaming ? getCachedMarkdownBlocks(text, imageMode) : null;
-      block.setAttribute('data-md-block', '');
+      if (cachedBlocks) {
-      // `display:contents` keeps margin-collapsing/spacing identical to a flat
+        let hasMermaidBlock = false;
-      // HTML body — the wrapper exists only for per-block reconciliation.
+        for (const cachedBlock of cachedBlocks) {
-      block.style.display = 'contents';
+          const block = document.createElement('div');
-      block.innerHTML = renderMarkdownSync(text, imageMode);
+          block.setAttribute('data-md-block', '');
-      // Decorate synchronously too: wrap code blocks in their framed card,
+          block.style.display = 'contents';
-      // mark inline code, build table controls, etc. The async pass re-decorates
+          block.innerHTML = cachedBlock.html;
-      // its own DOM before morphing, so without this the first paint shows bare
+          decorateMarkdown(block, ctx);
-      // <pre>/tables that "snap" into their decorated form a tick later. Matching
+          block.setAttribute('data-md-id', cachedBlock.id);
-      // the structure here keeps the async morph to syntax colors only.
+          block.setAttribute(MARKDOWN_DECORATION_ID_ATTR, decorationId);
-      decorateMarkdown(block, ctx);
+          hasMermaidBlock ||= shouldRefreshMermaidViewers(block);
-      target.appendChild(block);
+          target.appendChild(block);
-      if (shouldRefreshMermaidViewers(block)) {
+        }
-        refreshMermaidViewers();
+        if (hasMermaidBlock) refreshMermaidViewers();
+      } else {
+        const block = document.createElement('div');
+        block.setAttribute('data-md-block', '');
+        block.style.display = 'contents';
+        block.innerHTML = renderMarkdownSync(text, imageMode);
+        decorateMarkdown(block, ctx);
+        block.setAttribute(MARKDOWN_DECORATION_ID_ATTR, decorationId);
+        target.appendChild(block);
+        if (shouldRefreshMermaidViewers(block)) refreshMermaidViewers();
       }
+    } else if (!mermaidViewerRef.current && shouldRefreshMermaidViewers(target)) {
+      // StrictMode re-runs this setup after the cleanup probe. The DOM remains,
+      // but the viewer registry does not, so recreate it without reinstalling
+      // or re-decorating ordinary blocks.
+      refreshMermaidViewers();
     }
-  }, [containerRef, text, imageMode, ctx, refreshMermaidViewers]);
+  }, [containerRef, text, streaming, imageMode, ctx, refreshMermaidViewers]);
 
   React.useEffect(() => () => {
     mermaidViewerRef.current?.cleanup();
@@ -810,14 +842,12 @@ const useMorphdomMarkdown = ({
     if (!container) return;
     const target = container.querySelector<HTMLElement>('[data-markdown-content]') ?? container;
     let active = true;
+    const decorationId = getMarkdownDecorationId(ctx);
 
     void renderMarkdownBlocks(text, streaming, imageMode).then((blocks) => {
       if (!active) return;
       const existing = Array.from(target.children) as HTMLElement[];
 
-      // Reconcile per block: only re-morph blocks whose content changed, leaving
-      // stable leading blocks untouched. Keeps per-stream-step DOM work bounded
-      // to the trailing (growing) block instead of the whole message.
       blocks.forEach((block, index) => {
         let el = existing[index];
         if (!el) {
@@ -826,7 +856,28 @@ const useMorphdomMarkdown = ({
           el.style.display = 'contents';
           target.appendChild(el);
         }
-        if (el.getAttribute('data-md-id') === block.id) return;
+        if (el.getAttribute('data-md-id') === block.id) {
+          if (el.getAttribute(MARKDOWN_DECORATION_ID_ATTR) !== decorationId) {
+            const hasMermaidBlock = shouldRefreshMermaidViewers(el);
+            if (hasMermaidBlock) {
+              mermaidViewerRef.current?.cleanup();
+              mermaidViewerRef.current = null;
+            }
+            const replacement = document.createElement('div');
+            replacement.setAttribute('data-md-block', '');
+            replacement.style.display = 'contents';
+            replacement.innerHTML = block.html;
+            decorateMarkdown(replacement, ctx);
+            replacement.setAttribute('data-md-id', block.id);
+            replacement.setAttribute(MARKDOWN_DECORATION_ID_ATTR, decorationId);
+            el.replaceWith(replacement);
+            if (hasMermaidBlock || shouldRefreshMermaidViewers(replacement)) refreshMermaidViewers();
+          }
+          if (!mermaidViewerRef.current && shouldRefreshMermaidViewers(el)) {
+            refreshMermaidViewers();
+          }
+          return;
+        }
 
         const temp = document.createElement('div');
         temp.innerHTML = block.html;
@@ -838,12 +889,12 @@ const useMorphdomMarkdown = ({
           onBeforeElUpdated: (fromEl, toEl) => !fromEl.isEqualNode(toEl),
         });
         el.setAttribute('data-md-id', block.id);
+        el.setAttribute(MARKDOWN_DECORATION_ID_ATTR, decorationId);
         if (hadMermaidBlock || tempHasMermaidBlock || shouldRefreshMermaidViewers(el)) {
           refreshMermaidViewers();
         }
       });
 
-      // Remove any trailing block elements no longer present.
       const hadMermaidBeforeTrailingCleanup = shouldRefreshMermaidViewers(target);
       let removedMermaidBlock = false;
       for (let i = existing.length - 1; i >= blocks.length; i -= 1) {
@@ -856,7 +907,6 @@ const useMorphdomMarkdown = ({
       if (removedMermaidBlock || (existing.length > blocks.length && hadMermaidBeforeTrailingCleanup)) {
         refreshMermaidViewers();
       }
-
     });
 
     return () => {

packages/ui/src/components/chat/markdown/markdownCore.test.ts

@@ -49,7 +49,10 @@ import { escapeRawMarkdownHtml, isLocalFileUrl, MARKDOWN_FORBIDDEN_TAGS } from '
 const {
   __markdownImageCandidateCacheForTests,
   extractMarkdownImageCandidates,
+  getCachedMarkdownBlocks,
+  renderMarkdownBlocks,
   renderMarkdownSync,
+  resetMarkdownHtmlCacheForTests,
 } = await import('./markdownCore');
 const { resolveMarkdownImageSource } = await import('./markdownImageAssets');
 
@@ -90,6 +93,47 @@ describe('markdown sanitization', () => {
 
 });
 
+describe('Markdown block cache reads', () => {
+  test('returns all settled blocks synchronously after a full cache hit', async () => {
+    resetMarkdownHtmlCacheForTests();
+    const text = '**cached** settled markdown';
+
+    expect(getCachedMarkdownBlocks(text)).toBeNull();
+    const rendered = await renderMarkdownBlocks(text, false);
+
+    expect(getCachedMarkdownBlocks(text)).toEqual(rendered);
+  });
+
+  test('returns null for a cold or partial settled miss', async () => {
+    resetMarkdownHtmlCacheForTests();
+    const first = 'first settled block';
+    const changed = 'first settled block\n\nsecond settled block';
+
+    await renderMarkdownBlocks(first, false);
+
+    expect(getCachedMarkdownBlocks(changed)).toBeNull();
+  });
+
+  test('keeps image mode identity out of the settled full hit', async () => {
+    resetMarkdownHtmlCacheForTests();
+    const text = '![image](https://example.test/image.png)';
+
+    await renderMarkdownBlocks(text, false, 'inline');
+
+    expect(getCachedMarkdownBlocks(text, 'label')).toBeNull();
+    expect(getCachedMarkdownBlocks(text, 'inline')).not.toBeNull();
+  });
+
+  test('does not treat streaming live-cache entries as settled full hits', async () => {
+    resetMarkdownHtmlCacheForTests();
+    const text = 'streaming markdown';
+
+    await renderMarkdownBlocks(text, true);
+
+    expect(getCachedMarkdownBlocks(text)).toBeNull();
+  });
+});
+
 describe('Markdown images', () => {
   test('renders assistant images as icon-ready text without loading the source', () => {
     const html = renderMarkdownSync([

packages/ui/src/components/chat/markdown/markdownCore.ts

@@ -548,10 +548,30 @@ export const __markdownBlockCacheSizesForTests = (): { full: number; live: numbe
   live: liveBlockCache.size,
 });
 
-const parseBlock = async (
+/**
-  block: MarkdownBlock,
+ * Read a settled render synchronously when every block is already in the full
-  imageMode: MarkdownImageMode,
+ * cache. Cache reads retain the existing LRU `get` semantics and do not insert
-): Promise<string> => {
+ * or expand either cache.
+ */
+export const getCachedMarkdownBlocks = (
+  text: string,
+  imageMode: MarkdownImageMode = 'inline',
+): RenderedBlock[] | null => {
+  if (!text) return [];
+
+  const blocks = streamBlocks(text, false);
+  const rendered: RenderedBlock[] = [];
+  for (const block of blocks) {
+    const contentHash = contentFingerprint(block.raw);
+    const id = markdownBlockCacheKey(contentHash, block.mode, block.highlight, imageMode);
+    const html = fullBlockCache.get(id);
+    if (html === undefined) return null;
+    rendered.push({ id, html });
+  }
+  return rendered;
+};
+
+const parseBlock = async (block: MarkdownBlock, imageMode: MarkdownImageMode): Promise<string> => {
   const parser = imageMode === 'label' ? imageLabelParser : inlineImageParser;
   const parsed = await Promise.resolve(parser.parse(block.src));
   const withMath = renderMathExpressions(parsed);