What (not-well-known and not-often-seen) theorem is dynamically being illustrated in the applet below? [br](Feel free to move the triangle's vertices wherever you'd like.)
For a complete description of this theorem involving a triangle's excircle, see this problem-set problem posted in this [url=https://twitter.com/gogeometry/status/834149686533570565]tweet from Antonino Gutierrez[/url].