feat(toolbar): copy diagram to clipboard as PNG

Add a "⧉ Copy" button that places the current diagram on the system clipboard
as a PNG via the async Clipboard API (ClipboardItem). The write is invoked
synchronously with the PNG handed over as a promise, so it stays inside the
click gesture WebKit requires.

PNG export and clipboard copy now re-render through renderRaster(), which
forces htmlLabels off via an injected init directive. Flowcharts otherwise emit
<foreignObject> HTML labels that taint the canvas and make toBlob()/clipboard
writes fail — this also fixes pre-existing flowchart PNG export. The live
preview and SVG export keep the richer foreignObject labels.

Verified end-to-end: renderRaster yields a foreignObject-free SVG, it
rasterizes without tainting, and clipboard.write resolves on a real click.

README.md

@@ -28,7 +28,9 @@ TypeScript** frontend using **CodeMirror 6** and **Mermaid 11**.
   **focus mode** spotlights any node and its neighbours so you can read a dense
   graph without changing it. Every rewrite lands in the editor and is reversible
   with ⌘Z.
-- **Export** the current diagram to **SVG** or **PNG** (2× scale).
+- **Export** the current diagram to **SVG** or **PNG** (2× scale), or **copy a
+  PNG straight to the clipboard**. (PNG/clipboard re-render flowcharts with
+  text labels so the bitmap rasterizes cleanly across platforms.)
 - **Project registry** — recently opened projects are remembered in a local
   SQLite database so you can jump back in from the start screen.
 - **Themes** — switch the Mermaid theme (default / neutral / dark / forest /

src/lib/components/Toolbar.svelte

@@ -1,8 +1,18 @@
 <script lang="ts">
   import { store } from '../store.svelte';
-  import { exportSvg, exportPng } from '../export';
+  import { exportSvg, exportPng, copyPng } from '../export';
+  import { renderRaster } from '../mermaid';
   import type { MermaidTheme } from '../types';
 
+  // PNG and clipboard need a raster-safe SVG (text labels, not foreignObject),
+  // so re-render the current source rather than reuse the rich preview SVG.
+  async function rasterSvg(): Promise<string> {
+    const r = await renderRaster(store.content, store.theme);
+    if ('error' in r) throw new Error(r.error);
+    if (!r.svg) throw new Error('Nothing to render yet');
+    return r.svg;
+  }
+
   const themes: MermaidTheme[] = ['default', 'neutral', 'dark', 'forest', 'base'];
 
   async function doExport(kind: 'svg' | 'png') {
@@ -14,13 +24,28 @@
     try {
       const ok =
         kind === 'svg'
-          ? await exportSvg(store.lastSvg, name)
+          ? await exportSvg(store.lastSvg, name) // vector: keep rich labels
-          : await exportPng(store.lastSvg, name);
+          : await exportPng(await rasterSvg(), name);
       if (ok) store.notify(`Exported ${kind.toUpperCase()}`, 'success');
     } catch (e) {
       store.notify(e instanceof Error ? e.message : String(e), 'error');
     }
   }
+
+  let copying = $state(false);
+  function doCopy() {
+    if (!store.lastSvg) {
+      store.notify('Nothing to copy yet', 'error');
+      return;
+    }
+    copying = true;
+    // Call copyPng synchronously (passing the render as a promise) so the
+    // clipboard write stays inside the click gesture WebKit requires.
+    copyPng(rasterSvg())
+      .then(() => store.notify('Copied PNG to clipboard', 'success'))
+      .catch((e) => store.notify(e instanceof Error ? e.message : String(e), 'error'))
+      .finally(() => (copying = false));
+  }
 </script>
 
 <header class="toolbar">
@@ -63,6 +88,13 @@
     <div class="export">
       <button onclick={() => doExport('svg')} disabled={!store.lastSvg}>Export SVG</button>
       <button onclick={() => doExport('png')} disabled={!store.lastSvg}>PNG</button>
+      <button
+        onclick={doCopy}
+        disabled={!store.lastSvg || copying}
+        title="Copy PNG to clipboard"
+      >
+        {copying ? 'Copying…' : '⧉ Copy'}
+      </button>
     </div>
 
     <button

src/lib/export.ts

@@ -29,7 +29,7 @@ function loadImage(url: string): Promise<HTMLImageElement> {
   });
 }
 
-async function svgToPng(svg: string, scale: number): Promise<Uint8Array> {
+async function svgToPngBlob(svg: string, scale: number): Promise<Blob> {
   const { w, h } = svgDimensions(svg);
   const sized = withExplicitSize(svg, w, h);
   const blob = new Blob([sized], { type: 'image/svg+xml;charset=utf-8' });
@@ -44,15 +44,19 @@ async function svgToPng(svg: string, scale: number): Promise<Uint8Array> {
     ctx.fillStyle = '#ffffff';
     ctx.fillRect(0, 0, canvas.width, canvas.height);
     ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
-    const pngBlob: Blob = await new Promise((res, rej) =>
+    return await new Promise<Blob>((res, rej) =>
       canvas.toBlob((b) => (b ? res(b) : rej(new Error('PNG encode failed'))), 'image/png')
     );
-    return new Uint8Array(await pngBlob.arrayBuffer());
   } finally {
     URL.revokeObjectURL(url);
   }
 }
 
+async function svgToPng(svg: string, scale: number): Promise<Uint8Array> {
+  const blob = await svgToPngBlob(svg, scale);
+  return new Uint8Array(await blob.arrayBuffer());
+}
+
 export async function exportSvg(svg: string, name: string): Promise<boolean> {
   const path = await save({
     defaultPath: `${name}.svg`,
@@ -73,3 +77,20 @@ export async function exportPng(svg: string, name: string, scale = 2): Promise<b
   await api.writeBinaryFile(path, Array.from(bytes));
   return true;
 }
+
+/**
+ * Rasterize the diagram and place it on the system clipboard as a PNG image.
+ *
+ * Accepts the SVG as a value or a promise: the PNG `Blob` is handed to
+ * `ClipboardItem` as a promise and `clipboard.write` is invoked synchronously,
+ * so WebKit still treats the write as part of the originating click gesture
+ * (required) while awaiting the async render/rasterization.
+ */
+export async function copyPng(svg: string | Promise<string>, scale = 2): Promise<void> {
+  if (typeof ClipboardItem === 'undefined' || !navigator.clipboard?.write) {
+    throw new Error('Copying images to the clipboard is not supported here');
+  }
+  const blob = Promise.resolve(svg).then((s) => svgToPngBlob(s, scale));
+  const item = new ClipboardItem({ 'image/png': blob });
+  await navigator.clipboard.write([item]);
+}

src/lib/mermaid.ts

@@ -1,5 +1,6 @@
 import mermaid from 'mermaid';
 import elkLayouts from '@mermaid-js/layout-elk';
+import { mergeInit } from './optimize';
 
 // Register the ELK layout engine so diagrams (and the Optimize panel's "Switch
 // to ELK layout" fix) can request `layout: elk` for far cleaner dense graphs.
@@ -68,3 +69,17 @@ export async function renderMermaid(code: string, theme: string): Promise<Render
     document.getElementById(`d${id}`)?.remove();
   }
 }
+
+/**
+ * Render an SVG suitable for raster export (PNG / clipboard).
+ *
+ * Flowcharts render labels as `<foreignObject>` HTML by default, which taints
+ * the `<canvas>` they're drawn onto and makes `toBlob()` / clipboard image
+ * writes fail. Forcing `htmlLabels: false` (per-render, via an injected init
+ * directive — global preview config stays rich) emits plain `<text>` labels
+ * that rasterize cleanly across engines. Other diagram types are unaffected.
+ */
+export function renderRaster(code: string, theme: string): Promise<RenderResult> {
+  if (!code.trim()) return Promise.resolve({ svg: '' });
+  return renderMermaid(mergeInit(code, { htmlLabels: false }), theme);
+}