Don't use rAF with WebGPU (and canvas in general)

So you want a pixel perfect rendering. You do a little bit of research and discover the ResizeObserver (RO) API with device-pixel-content-box. This enables setting of an integer number of screen pixels for images in canvas "swapchain".

What you also want is to run animations with a speed of screen refresh rate, using a familiar requestAnimationFrame(rAF).

Challenges arise in combining both APIs, each case presenting a downside:

• Rendering in rAF while resizing in RO results in an undesirable black frame effect.
• Rendering in rAF, resizing in RO, and rendering again resolves the issue but at the cost of rendering it twice!
• Attempting to optimize by saving size in RO for the next rAF introduces a one-frame delay on resize. The canvas image appears stretched during resizing.

As we can see, layout phase in browsers happens after the rAF, therefore we can't know in advance that size will change and suspend the rendering in order to do it in the ResizeObserver callback instead.

Now, consider to stop using rAF for animations completely.
ResizeObserver has all the properties we want from rAF:

• Operates at the screen refresh rate.
• Pauses execution when the browser tab is invisible.
• Allows queuing for subsequent runs within its callback, even when no resizing is occurring.

So instead of conventional loop

function rafLoop() {
  device.queue.submit([cmdBuffer.finish()]);
  requestAnimationFrame(rafLoop);
}
requestAnimationFrame(rafLoop);

Do a ResizeObserver based loop

const observer = new ResizeObserver(roLoop);

function roLoop(resizeEntries: ResizeObserverEntry[]) {
  if (canvasEl.size !== resizeEntries[0]) {
    canvasEl.size = resizeEntries[0];
    // resize depth texture, etc
  }

  device.queue.submit([cmdBuffer.finish()]);

  // unobserve to trigger the callback
  observer.unobserve(canvasEl);
  observer.observe(canvasEl, { box: "device-pixel-content-box" });
}
observer.observe(canvasEl, { box: "device-pixel-content-box" });

That's it. Now we have a perfect rendering during resizing.