backgroundSize was added in 6.4?

Good catch! Updated in 6a82e04. The output of backgroundSize was added in 6.4 but the UI controls weren't. Since this function already handled output for backgroundSize, I've updated the comment here to just reflect position and repeat output.

Could we check if a background image url exists as part of the condition that checks for block support above? That way we could return early if there is no image. Or are there reasons to still go through this logic in the absence of a url?

We could do an early return. It was mostly laid out like it currently is to support subsequent updates where we might have a range of different values for the background image source. I.e.

• 'featured_image' === $background_image_source
• 'id' === $background_image_source (for referencing a media library item directly), etc

So we could do an early return just before we define $background_size, something like:

if ( ! $background_image_source && ! $background_image_url ) {
  return $block_content;
}

Would that be worth doing, do you think?

Wouldn't those sources still require defining a URL though? We always need the URL in order to output the styles. But yeah I guess in any case, in subsequent updates we can add to the condition if that changes.

Edit: to be clear, I do think it's worth returning as early as possible 😄

Okie-doke! I'll update with an early return 👍

For the featured_image case we won't have a url in the block attributes, as that would get generated at render time. But that could happen anywhere we like in the function, so we could always put that logic before this $background_position line and do an early return if we don't have a featured image, either. Let's take a look at that part when we go to implement the feature 🙂

Added an early return in c2982a3 — let me know if you'd like me to tweak it!