Collapsible hats/c-blocks

In advance, if it's not appropriate for me to bring this up here, I apologize, close this topic, and I'll be on my way. However, this seems like a pretty important design consideration.

One of the biggest UI/UX issues in using almost any visual programming environment I've encountered (such as Scratch, or Mindstorms NXT-G/EV3-G) is that the code can get extremely long and (arguably) cluttered. Even in a text-based environment, length can still be an issue so code blocks are often able to be collapsed in editors, something which seems to make a lot of sense for Scratch/Blockly (vertical specifically).

This has been brought up a few times (the core thread on the forums being this one), and this potentially fits quite well into Scratch by implementing this within hats and c-blocks, by including an arrow to allow all code to be effectively hidden from the immediate view, meaning that it's significantly easier to get an overview of the sections of code. I imagine this would be important to consider as this can have a number of implications (such as how blocks are moved once 'extended', room in block length for an arrow, etc).

Thoughts are appreciated.

No files, tests, or entry points are identified in the issue. Start with the linked Scratch forum discussion and review how hats and C-blocks currently behave; this issue is done only after the collapse interaction and its layout implications have an agreed design and implementation.