Navigation Block: replace button block appender with dropdown appender

[t-hamano]

Related issue/PR:

• Navigation block: missing inline + inserter to add new menu items without selecting parent block #37572
• Navigation block: fix block appender size #66153

The current Navigation block uses a button block appender and has some override CSS in order to show the block appender even when a child is selected.

But as seen in this comment, we should use a dropdown appender instead of a button block appender as it's only a temporary fix.

That way you should be able to remove a lot of the hacky CSS.

[im3dabasia]

@t-hamano ,

I noticed that the Group block also uses a button block appender, similar to the Navigation block. Am I missing something here? It seems the goal is to move away from the button block appender, but it looks like the Group block isn't currently using the dropdown appender.

Navigation

gutenberg/packages/block-library/src/navigation/edit/inner-blocks.js

Lines 100 to 102 in 8689580

	parentOrChildHasSelection
	? InnerBlocks.ButtonBlockAppender
	: false,

Group

gutenberg/packages/block-library/src/group/edit.js

Lines 120 to 122 in 8689580

	// use the larger button appender.
	renderAppender = InnerBlocks.ButtonBlockAppender;
	}

[t-hamano]

I noticed that the Group block also uses a button block appended

This is only rendered when the Group block does not have any child blocks:

If child blocks exist, rendered as a drop-down appender: