Always visible appender is inconsistently visible, causing visual offsets

[jasmussen]

#15864 changed the behavior of when the side inserter is showing. But it is showing inconsistently, and in many nesting situations the presence makes things offset. Here's a simple example:

That's a list (i.e. not paragraph) transformed into a group. Suddenly there's a blank space in the editor that won't be there on the frontend.

This is also the case in other blocks that support nesting, such as Cover, Columns, Media & Text: if the last block isn't a paragraph, there's an empty space.

Here's a columns block with three images:

[ZebulanStanphill]

Related: #13571.

[richtabor]

I second this here. The inserter within the Group block should perform as the inserter within the singular Column block does. One change would be that the inserter returns when you're selected on a block.

Otherwise, the Group block within the editor does not properly resemble the front-end when you're not interacting with it.

[jasmussen]

The reason for adding this trailing inserter to nesting containers makes sense in principle, but I wonder if we can tweak the behavior so it's invisible until the block itself is selected?

Something along the lines of this:

.block-editor-block-list__block .block-editor-default-block-appender + .block-editor-default-block-appender {
	display: none;

	.is-selected & {
		display: block;
	}
}

The idea being that this leans into the principle of the unselected block is a preview, whereas the selected block can have additional UI.

[jasmussen]

Created a PR with a fix in #16453.