Legally they can't even use those, React does a totally blackbox reimplementation

You don't have to use a clean room methodology. Decompiling the code and reimplementing the functionality is a perfectly valid approach in any jurisdiction that permits reverse engineering.

Wait, pardon my ignorance, but isn't a decompiler required for the kind of work they do at react?

Person A's Job:

- Decompile shit.

- Then write down the names of the functions with (1) input, (2) output, (3) a description of what person a think the code is doing (4) any side effect / preconditions / post conditions they can deduce.

Person B's Job:

- Take the spec created by person A and write code.

while(missingFunctionality.hasNext): goto Person A's Job

It is and the split between person A doing the first part and person B doing the second part is important in a "clean room" reimplementation in the US.

A lot of stuff is based or observing the Windows functionality in a debugger, or Microsoft's API documentation.

decompiler vs. disassembler is an important distinction here

The source I heard this in was this video at 4:40 — https://www.youtube.com/watch?v=2D9ExVc0G10

A source: https://news.ycombinator.com/item?id=20341022