Double inserter button showing at the end of some block lists

[talldan]

Description

In some block lists (like the navigation block) the trailing appender shows as well as a sibling inserter.

This was introduced by #28418. The solution is probably that the insertion point can be displayed after the last block when hovering over blocks in the block library or quick inserter menu. It shouldn't display when there's a trailing appender already.

Step-by-step reproduction instructions

1. Add a navigation block
2. Add some menu items
3. Hover around the trailing edge of the last menu item

Expected behaviour

Only one inserter

Actual behaviour

It's very insertery

Screenshots or screen recording (optional)

ghost-inserter.mp4

[jom]

This seems to happen [also?] with InnerBlocks that have its own appender.

[ocean90]

Just noticed the same with a custom block with InnerBlocks which only allows one block via allowedBlocks and uses renderAppender: () => <InnerBlocks.ButtonBlockAppender />. It's also odd that using the additional appender sometimes if not always adds the block to the first position.

@youknowriad Do you have some ideas how we could fix this?

[talldan]

#29920 should fix it.

[ocean90]

Added to the WordPress 5.7.x project, would be nice to include the fix in a minor update.