Social Icon: Add variation placeholder state

[Mamaduka]

Based on the discussion at #59162 (comment).

A proposal to introduce a placeholder state for the Social Icon/Link block when no variation is selected - the service attribute is undefined. Currently, it defaults to a Link (chain) variation.

The placeholder will let the user choose the block variation.

Why?

This will allow the editor to programmatically insert the block without variation and let the user pick the desired one.

Screenshot

A group block placeholder.

[Mamaduka]

An alternative solution could be opening the main inserter for blocks that have a single child with many variations. You can see a dome here - #29490 (comment).

The approach could also resolve #46753.

@jasmussen and @richtabor, I would appreciate your input here.

[jasmussen]

Thanks for the issue, thanks for the ping!

I would avoid the big white/black-bordered placeholder pattern. The problem with that one is it doesn't scale well to narrow contexts, like if you're inserting social links into a Navigation block. It also causes layout shifts — the row of icons will nearly always be smaller than this setup state.

I quite liked what Rich did here:

social.mp4

If we went that direction, I even think we could move towards a place where Social Links wasn't a container block with individual icons inside, but just a single block with multiple icon transforms. This would vastly simplify inserting social links in other contexts, including the navigtion block. What do you think?

[Mamaduka]

I used the Group placeholder as rough example. A small + button will be more suitable here.

What do you think an alternative approach I shared in the latest comment - #59190 (comment)?

[jasmussen]

What do you think an alternative approach I shared in the latest comment - #59190 (comment)?

This still requires a social links container, no? I'd love to find a pattern that would let us have a single "Social Link" block that could hold the full interface, so that it would work in any context. That's not to say the container couldn't remain there, it's useful to style colors across — just that mostly abstracting it away seems like it would help the majority of pattern and building cases.

[Mamaduka]

@jasmussen, let's start with the ideas you and Rich provided in #56259. I have a feeling after it's implemented, we can also close this issue.