Euclid's Proof of the Pythagorean Theorem Proof
(a.k.a. “The Peacock”,“The Windmill”, or “The Bride's Chair”)
This applet is labeled the same way as the Euclid's proof in Sir T. Heath's translation:
https://www.cut-the-knot.org/pythagoras/Proof1.shtml (Cut the Knot website)