Spacer block: Can't grab the block's height adjuster

[mapk]

Describe the bug
I can't grab the block's height adjuster due to the block below showing the Insert block appender over it.

To reproduce
Steps to reproduce the behavior:

1. Add a Spacer block.
2. Add another block below it.
3. Select the Spacer block again.
4. Try moving the cursor to the node on the bottom of the Spacer block to adjust its height.
5. Observe the Block Inserter appender appearing preventing you from grabbing the node.

Expected behavior
I expected to be able to grab the node to the adjuster without it being hidden under the block appender.

Screenshots

Desktop (please complete the following information):

• OSX
• Firefox 68.0
• Gutenberg 6.1

[mtias]

This is pretty annoying.

[enriquesanchez]

Discussed during today's Design triage in Slack.

The team agreed that this was a larger issue around the placement of the appender. A couple of suggestions came up:

1. Have the appender appear in between two blocks when you hover long enough, moving them slightly apart.

2. Have the appender positioned just outside the main block UI so it doesn't overlap any of the block's UI. Quick mockup:

Concerns about accessibility and mobile support should be considered.

[mtias]

Let's try how a delay of 2s or so feels for this. It might give a decent baseline since it'd appear when you want to, but not be in the way during most common operations.

[shaunandrews]

I worked up a mockup showing what it could be like to only show the inserter after hovering for a second, and then moving blocks out of the way to make room:

[shaunandrews]

Here's another iteration on the animation above. This time, I made the line thinner and had it transition in earlier than the + icon.

[jasmussen]

Looks great, Shaun.

A question will likely surface whether this makes it harder to get to the sibling inserter. Which it might. But at the same time it also makes the act of inserting between two blocks more intentional and less fiddly, and opens up blocks like #17143 for happening. In that vein, I would suggest this is a fine tradeoff to make.

What's the next step to get this shipping? I'll juggle some labels.

[shaunandrews]

this makes it harder to get to the sibling inserter.

...and...

makes the act of inserting between two blocks more intentional and less fiddly.

Exactly! This does make it harder to find the sibling inserter — and we may want to look at adding a tip about this somewhere — but it also makes the pattern a little more intentional and understandable once learned. It also reduces the amount of "stuff" that pops in-and-out when moving a cursor around a page with lots of blocks.

[kjellr]

I'm going to reopen this one. #17136 was an improvement, but it still requires some very careful mousework to differentiate between the resizer and the inserter.

[mtias]

This seems to be working fairly well now with the latest design adjustments. There's probably some further improvements that could be done to the in-between inserter in general.