toDataURL returns a black image, Cesium included

• The export button produces a 1920x1080 pure black PNG.
• Some browsers throw Uncaught DOMException: Failed to execute toDataURL on HTMLCanvasElement: Tainted canvases may not be exported.
• The same capture code works on a page that grabs the frame synchronously.

1. Call toDataURL directly inside the click handler to separate timing from cross-origin.
2. Read the error text: Tainted canvases means CORS, silence means an empty buffer.
3. Look for setTimeout, await or requestAnimationFrame between the render and the capture.
4. Check whether tile responses carry Access-Control-Allow-Origin.

Pass contextOptions with webgl preserveDrawingBuffer true and alpha false to the Viewer, or capture inside scene.postRender after requestRender. For tainted canvases add Access-Control-Allow-Origin on the tile server.

Uncaught DOMException: Failed to execute 'toDataURL' on 'HTMLCanvasElement':
Tainted canvases may not be exported.

var viewer = new Cesium.Viewer('cesiumContainer', {
  contextOptions: {
    webgl: {
      preserveDrawingBuffer: true,
      alpha: false
    }
  }
});

• Enabling preserveDrawingBuffer on every page, which costs a buffer copy per frame and extra GPU memory on mobile.
• Screenshotting from a setTimeout without preserveDrawingBuffer, by which point the buffer has already been cleared.