diff --git a/frontend/src/canvas/MarkdownOverlay.ts b/frontend/src/canvas/MarkdownOverlay.ts
new file mode 100644
index 0000000..8cb8af7
--- /dev/null
+++ b/frontend/src/canvas/MarkdownOverlay.ts
@@ -0,0 +1,200 @@
+/**
+ * MarkdownOverlay — manages DOM div elements positioned over the PixiJS canvas
+ * for each visible markdown card.
+ *
+ * Creates a container div for each markdown item, positioned via viewport.toScreen().
+ * React components (MarkdownReadView / MarkdownEditView) render inside these containers.
+ * Handles culling (off-screen cards removed), zoom scaling, and position sync for
+ * drag/resize/remote transforms.
+ */
+
+import type { Viewport } from 'pixi-viewport';
+import type { SceneManager, SceneItem } from './SceneManager';
+import type { MarkdownObject } from './scene-format';
+import { MarkdownSprite } from './sprites/MarkdownSprite';
+import { extractTitle, MD_MAX_CONTENT_LENGTH } from './markdownDefaults';
+
+interface CardEntry {
+  /** The root DOM div for this card, positioned absolutely over the canvas. */
+  el: HTMLDivElement;
+  /** The content mount point inside el where React renders read/edit views. */
+  contentMount: HTMLDivElement;
+  /** Current item ID. */
+  id: string;
+}
+
+export class MarkdownOverlay {
+  private _viewport: Viewport;
+  private _scene: SceneManager;
+  private _container: HTMLElement;
+  private _cards: Map<string, CardEntry> = new Map();
+  private _editingId: string | null = null;
+
+  /** Callback when a card's height changes (from DOM measurement). */
+  onHeightChange: ((id: string, newHeight: number) => void) | null = null;
+
+  /** Callback to enter edit mode for a card (wired by Editor.tsx). */
+  onRequestEdit: ((id: string) => void) | null = null;
+
+  /** Callback when checkbox is toggled in read mode. */
+  onCheckboxToggle: ((id: string, newContent: string) => void) | null = null;
+
+  /** Callback when visible card set changes (cards added/removed). Used by React to sync portals. */
+  onChange: (() => void) | null = null;
+
+  constructor(viewport: Viewport, scene: SceneManager, container: HTMLElement) {
+    this._viewport = viewport;
+    this._scene = scene;
+    this._container = container;
+
+    this._onViewportMoved = this._onViewportMoved.bind(this);
+    viewport.on('moved', this._onViewportMoved);
+    viewport.on('zoomed', this._onViewportMoved);
+  }
+
+  /** Get the DOM mount point for a card (used by React to portal into). */
+  getMountPoint(id: string): HTMLDivElement | null {
+    return this._cards.get(id)?.contentMount ?? null;
+  }
+
+  /** Set which card is in edit mode (null = none). */
+  setEditing(id: string | null): void {
+    this._editingId = id;
+    // Update pointer-events: edit card gets auto, others get none
+    for (const [cardId, entry] of this._cards) {
+      entry.el.style.pointerEvents = cardId === id ? 'auto' : 'none';
+    }
+  }
+
+  get editingId(): string | null {
+    return this._editingId;
+  }
+
+  /** Full refresh — recreate all visible cards. Called on scene load, undo/redo. */
+  refreshAll(): void {
+    const items = this._scene.items;
+    const visibleIds = new Set<string>();
+
+    for (const [id, item] of items) {
+      if (item.type !== 'markdown') continue;
+      if (!item.data.visible) continue;
+
+      // Culling: skip items outside viewport bounds
+      if (!this._isVisible(item)) continue;
+
+      visibleIds.add(id);
+
+      if (!this._cards.has(id)) {
+        this._createCard(item);
+      }
+      this._positionCard(id);
+    }
+
+    // Remove cards that are no longer visible
+    for (const [id] of this._cards) {
+      if (!visibleIds.has(id)) {
+        this._removeCard(id);
+      }
+    }
+  }
+
+  /** Update a single card's position (for drag, resize, remote transform). */
+  updateItem(id: string): void {
+    const item = this._scene.getById(id);
+    if (!item || item.type !== 'markdown') return;
+
+    if (!this._isVisible(item)) {
+      this._removeCard(id);
+      return;
+    }
+
+    if (!this._cards.has(id)) {
+      this._createCard(item);
+    }
+    this._positionCard(id);
+  }
+
+  /** Remove a card's overlay (item deleted). */
+  removeItem(id: string): void {
+    this._removeCard(id);
+  }
+
+  /** Measure a card's DOM height and update data.h + sprite. */
+  measureHeight(id: string): void {
+    const entry = this._cards.get(id);
+    const item = this._scene.getById(id);
+    if (!entry || !item || item.type !== 'markdown') return;
+
+    requestAnimationFrame(() => {
+      const h = entry.el.offsetHeight / this._viewport.scale.x;
+      if (Math.abs(h - item.data.h) > 1) {
+        item.data.h = h;
+        // Redraw MarkdownSprite background
+        if (item.displayObject instanceof MarkdownSprite) {
+          item.displayObject.updateFromData(item.data as MarkdownObject);
+        }
+        this.onHeightChange?.(id, h);
+      }
+    });
+  }
+
+  destroy(): void {
+    this._viewport.off('moved', this._onViewportMoved);
+    this._viewport.off('zoomed', this._onViewportMoved);
+    for (const [id] of this._cards) {
+      this._removeCard(id);
+    }
+  }
+
+  // -- Private --
+
+  private _onViewportMoved(): void {
+    this.refreshAll();
+  }
+
+  private _isVisible(item: SceneItem): boolean {
+    // pixi-viewport exposes bounds via .left/.right/.top/.bottom
+    const vp = this._viewport;
+    const d = item.data;
+    return !(d.x + d.w < vp.left || d.x > vp.right || d.y + d.h < vp.top || d.y > vp.bottom);
+  }
+
+  private _createCard(item: SceneItem): void {
+    const el = document.createElement('div');
+    el.style.position = 'absolute';
+    el.style.pointerEvents = item.id === this._editingId ? 'auto' : 'none';
+    el.style.transformOrigin = 'top left';
+    el.style.overflow = 'hidden';
+    el.dataset.markdownId = item.id;
+
+    const contentMount = document.createElement('div');
+    el.appendChild(contentMount);
+
+    this._container.appendChild(el);
+    this._cards.set(item.id, { el, contentMount, id: item.id });
+    this.onChange?.();
+  }
+
+  private _positionCard(id: string): void {
+    const entry = this._cards.get(id);
+    const item = this._scene.getById(id);
+    if (!entry || !item) return;
+
+    const data = item.data as MarkdownObject;
+    const zoom = this._viewport.scale.x;
+    const screen = this._viewport.toScreen(data.x, data.y);
+
+    entry.el.style.left = `${screen.x}px`;
+    entry.el.style.top = `${screen.y}px`;
+    entry.el.style.width = `${data.w}px`;
+    entry.el.style.transform = `scale(${zoom})`;
+  }
+
+  private _removeCard(id: string): void {
+    const entry = this._cards.get(id);
+    if (!entry) return;
+    entry.el.remove();
+    this._cards.delete(id);
+    this.onChange?.();
+  }
+}