Obviously this is very WIP (but still works).

For example, I have no idea how not to apply this functionality to desktop view.

Also, what happens in an ancient browser? https://caniuse.com/details

Very cool!

Not sure if we prefer to make isCollabsable a property of the base classes, instead of making them non-final - we have a lot of these kinds of properties going around. 🤷‍♂️

For ancient browsers, IIRC, the fallback is just to show both parts without the "details" section being collapsible. Curios to see how this renders though - is the toggle button pretty? Can it be customized?

There are 2 places where we differentiate "small screen" - most things happen just with CSS, but there's something that happens in the PHP code - I think Home has that (It used to only show the menu), and possibly Differential Revision page. I think there's no JS element that triggers on load and checks for width, but I could be wrong.
Naturally, stuff that happens in PHP isn't "responsive" for dynamically changing the viewport size.
But I also don't think it's that important not to enable this on Desktop? Assuming the default state is "open".

IMHO this is ready to review.

Only issue remaining is backwards compatibility for browsers not understanding <summary> and <details>. https://stackoverflow.com/questions/22644268/details-and-summary-tag-compatibility-issues was not too useful either.

See https://caniuse.com/details - YMMV but I'd say I do not want to care too much about folks still using IE11, Firefox 48, Chrome 11, Edge 18, Android browser 3, Safari 5.1, or Opera Mini.