Add ability to restrict block to only be used once within inner blocks area

[fabiankaegy]

What problem does this address?

Gutenberg currently allows you to mark a block so that it can only be allowed once per page. Which is very useful. However there are instances where you may want to allow a block to be used multiple times within the page but only once within a certain section.

If you for example have a "Navigation" block that it would be nice to only allow the "Login" button once per navigation. But still allow multiple navigation blocks per page.

What is your proposed solution?

I'm not sure what the best API for it would be. But somehow allowing a block to only be used once within a certain context that is different to the entire page.

[luisherranz]

Initially, this looks to me like a subcase of #42342 where the maximum number of blocks of a certain type is 1, instead of X.

@fabiankaegy: do you see different requirements?

[fabiankaegy]

@luisherranz i had not thought of it from top down. Instead I always looked at it bottom up which brought me to think of it similar to the supports.multiple that already exists.

I think the main need here is that as a developer I may want to say that the post author block for example should only be able to be inserted one time as an ancestor of my post template block.

(Just taking them as examples)

So it is a littel different to the need for a fixed set of inner blocks because the once per container feature should not just take the direkt child but any ancestor into account.