The browser is rendering that region of its window as transparent, and the display compositor is obliging by showing the layers below the browser window. The browser window itself doesn't know anything about the pixels of the layers below it. So anything that looks at the browser window, like canvas.getImageData will not return those pixels.

However screen-sharing API like getDisplayMedia() will screencopy at the compositor level, so they will include those pixels depending on what you choose to share.