srcset and sizes attributes missing from the featured image meta box

[johnbillion]

Describe the bug

When setting a featured image with the classic editor, the image in the meta box contains srcset and sizes attributes which means the potentially very large original image doesn't get displayed.

In Gutenberg these attributes are missing, meaning the original image is used and scaled down in the browser.

To reproduce

1. Go to Posts -> Add New
2. Upload a large image and set it as the featured image
3. Observe that the large original image file is used in the img tag in the Featured Image meta box.

Expected behavior

The same srcset and sizes attributes that the classic editor use should be present.

[subject026]

could I have a look at this?

[johnbillion]

@subject026 Sure, feel free, you don't need to ask!

[thomashigginbotham]

I submitted a pull request that resolves this issue. #29414

[skorasaurus]

The current implementation doesn't use srcset; but instead uses the registered_image_sizes to calculate which image size to detect - https://github.com/WordPress/gutenberg/blob/trunk/packages/editor/src/components/post-featured-image/index.js

Any install that has the default register sizes will pick a smaller image; as shown in this example using gutenberg 12.8.1.