Adjust transition and delay for the between-blocks inserter.

[mtias]

This aims to make the between-blocks inserter appear when needed, but out of the way when not. It reduces the visual weight a bit by removing the line. When opening the inserter and hovering a block, the usual blue line still appears.

[youknowriad]

I like this, especially the line removal

[jasmussen]

Love it.

Can you move the plus down a couple of pixels, or the blue insertion indicator up, so they align?

Otherwise 👍 👍

[mtias]

Tweaked the position a bit, which also fixed the focus halo:

[jasmussen]

Toot toot

[aduth]

Seems like the delay should be skipped when the button is activated by focus. Feels awkward when tabbing between blocks.

[aduth]

What was the reason for this change? Was it that it was too difficult to trigger the hover effect? The issue with this is that it aggravates the difficulty in selecting blocks which are short (like single-line paragraphs). The 44px was intended for hover effect only, intentionally overlapping some of the content of the block so that it would be slightly less easy to unhover the between-inserter (related). The natural height would be closer to 28px. At the very least, if we keep this, we need to remove the now-redundant hover style override:

gutenberg/editor/components/block-list/style.scss

Lines 415 to 417 in baa0ff7

	&:hover:before {
	height: 44px;
	}

[jasmussen]

I can't speak to the intent of this particular code, but wanted to point in the direction of #4312, in case it's relevant.

[aduth]

See #4384