This worked well in my testing across a variety of themes. The only issue I noticed was when inserting an image at the "large" size with a large image upload that results in a -scaled image, the jpeg is used for the image src, since the webp version isn't available yet (see #174).

Thanks for taking the time to test it, currently this is expected and described here:

Based on the decision from:

• When generating WebPs, ensure full size is generated(-scaled for large images) #174

The code would be updated accordingly if required.

@adamsilverstein

This worked well in my testing across a variety of themes. The only issue I noticed was when inserting an image at the "large" size with a large image upload that results in a -scaled image, the jpeg is used for the image src, since the webp version isn't available yet (see #174).

I noticed that as well, it depends on the viewport size and pixel density. Since we currently don't have WebP versions for the "full" size (whether -scaled or original), it's best to currently test this PR on a mobile viewport since there it's most likely to get served a WebP image.

The problem is that most modern screens have a high pixel density like @2x, which means if I look at an image of 600px (or more accurately in a viewport of 600px), the browser will look for a size >=1200px, which would already be more than WordPress core's large by default.

If you want to test this more specifically without the above caveats, you can put a temporary filter in place to ensure WordPress never chooses a size greater than the image size you actually specified (which may cause blurry images on larger or high-density devices though):

add_filter(
	'wp_calculate_image_srcset',
	function( $sources, $size_array ) {
		foreach ( $sources as $index => $source ) {
			if ( 'w' === $source['descriptor'] && $source['value'] > $size_array[0] ) {
				unset( $sources[ $index ] );
			}
		}
		return $sources;
	},
	10,
	2
);

There's certainly room for optimization in core's default behavior there, but that's a separate rabbit hole :)

Right, this answers that question for me - we need the full size webp available for the correct markup when users select the full size image.

I agree this gives us an answer for #174, that we should also generate a WebP version of the full size.

Thanks, @felixarntz just updated to code to follow what WP Core uses in order to reduce the friction in the scenario this would be merged into WP Core.

Let me know in case you have any additional feedback.