Allow expanding and collapsing of nested blocks in List View

[talldan]

Nested blocks can get quite complicated in List View. The list view shows the structure of your content. It is present in a few places, it's in the post editor, it works as a persistent sidebar in the site editor, and it exists as a tool for the navigation block.

Allowing users to expand and collapse parent blocks to reveal or hide the inner blocks would be a good feature.

What is your proposed solution?

Add a small chevron to the left of blocks that feature nesting:

Note: This change should apply to anywhere we use the List View component.

[jasmussen]

I took the liberty of updating this ticket with mockups from the duplicate #29726.

[gwwar]

I can take a look at this one, seems like we might be able to get some perf wins here too with the feature.

[gwwar]

https://www.w3.org/TR/wai-aria-practices/examples/treegrid/treegrid-1.html looks like a nice example/resource

[jasmussen]

On behalf of Kerry who did all the work, this one is now fixed! 🎉