Inserter: Always show blue line

[jasmussen]

In previous versions, a blue line would appear in the block list, where the block itself would be inserted:

This regressed recently, and that blue line is no longer available:

Let's both fix this, and revisit it. We should try showing this blue line as soon as you open the inserter. I.e. this:

So, not just on hovering a block in the inserter, but just by virtue of it being open.

[jasmussen]

@ellatrix I know you've worked on making the DOM lighter — do you have any idea if that might have caused the blue line to go missing?

[ellatrix]

@jasmussen There's aPR here: #20792