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Let canvas extensions capture the exact pixels shown in a browser preview #3184

Description

@sjur-gr

Feature summary

screenshot API for canvas extensions

What problem are you trying to solve?

Today, an extension can inspect or rebuild the page, but it cannot capture the actual rendered preview. Rebuilding it from the DOM gives different results for responsive layouts, fonts, canvas, video, WebGL, embedded content, and browser-rendered effects.

Proposed solution

Could the canvas provide a screenshot API that:

  • Captures the visible preview exactly as rendered.
  • Uses the current viewport size and device pixel ratio.
  • Captures compositor output instead of rebuilding the DOM.
  • Returns a PNG or blob to the extension.
  • Only gives an extension access to its own canvas.

I only need the visible preview, not a full-page screenshot or the surrounding Copilot UI. This would make visual annotation reliable and framework-independent.

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