The in-between inserter doesn't appear when the last child block is selected

[hanneslsm]

Description

Coming from #47200 (comment)

The inserter does appear in Galleries, even when an image is selected. However, it does not appear when the last image is selected.

Step-by-step reproduction instructions

1. Create a galley
2. Insert images
3. Select the last image
4. The inserter does not show up on the left and top of the images

Screenshots, screen recording, code snippet

Screen.Recording.2023-08-03.at.23.14.26.mp4

Environment info

• WP 6.3-RC3-56344
• GB 16.3

Please confirm that you have searched existing issues in the repo.

No

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[annezazu]

Thanks for opening this! Looks like a straight forward dev task to dive into. Added to the Polish board :)

[hanneslsm]

We must consider what happens if there is only one image in the gallery. Probably nothing, but if we find a solution it could fix #50021

[Mamaduka]

I believe this issue isn't limited to the Gallery block; I can reproduce the same bug with Buttons and Columns.

As far as I can tell, useInBetweenInserter returns early due to this condition:

gutenberg/packages/block-editor/src/components/block-list/use-in-between-inserter.js

Lines 132 to 136 in 02dc886

	// Don't show the inserter when hovering above (conflicts with
	// block toolbar) or inside selected block(s).
	if ( getSelectedBlockClientIds().includes( clientId ) ) {
	return;
	}

cc @ellatrix, @talldan

Steps to reproduce

1. Open a post or page.
2. Insert Buttons or Column blocks with at least three inner blocks.
3. Select the last block.
4. Try enabling in-between inserter. See the screencast.

Screencast

CleanShot.2024-02-13.at.17.37.44.mp4

[talldan]

I believe this issue isn't limited to the Gallery block; I can reproduce the same bug with Buttons and Columns.

As far as I can tell, useInBetweenInserter returns early due to this condition:

Good find, that stops the in-between inserter from showing under the toolbar in vertical block lists, but it wouldn't be an issue for horizontal block lists. There could be some kind of additional logic added to that if statement.