The "sibling inserter" is the so-far name for the interface we show when you hover between two blocks. This interface is there to add an affordance to insert content between two blocks. It is also an "escape hatch", because even though you could simply select the preceeding block and press "enter" to create a space for insertion, block authors might create a block that captured enter keypresses, trapping you.

The sibling inserter is not used on mobile.

A recent change to the sibling inserter meant you had to click a plus in the middle in order to insert a provisional linebreak where you can insert new content. This was meant to make inserting between blocks less error prone, compared to previously where clicking anywhere on the line between blocks would insert a provisional block. The latter would invoke constantly even when you didn't want it to. But the plus at the center is also not ideal, as it is often covered by the block toolbar:

Let's rethink how this could work.

One idea is to show the plus to the left of the side UI:

You can still hover between blocks, but you can only click on the left. This concept would need some work for responsive breakpoints, but could probably be made to work.

What other ideas are there for creating an intuitive interface to insert between blocks?

GIF of the current behavior: