Provide clear distinction when expanding/collapsing block categories

[jancavan]

Is your feature request related to a problem? Please describe.
I kind of lose track of where I am when expanding/collapsing block categories:

Describe the solution you'd like

1. Subtle animation when expanding/collapsing sections.
2. Perhaps the opened section should look slightly different from the collapsed ones.

Describe alternatives you've considered
I haven't explored any alternatives, but either one (or both?) solutions above will help.

[ZebulanStanphill]

Here is a mockup GIF from one of the comments in #6168 which shows one alternative solution for always keeping the current block category visible: make it sticky.

[jancavan]

The animation is nice and really helpful. Now that I'm looking at it, I still think there needs to be a clearer distinction between an expanded vs a collapsed section.

I know the arrows are supposed to indicate expansion, but they're quite small, and the lines between them are really faint. What about a different heading color for the expanded section to establish clearer boundaries? cc @shaunandrews

[karmatosed]

Closing to focus us on the micro-interactions issue as this is a great one to include: #8029