The red point is the intersection of the altitudes of the black triangle. It is called the orthocenter. This point may be moved and the triangle is updated to make the altitudes intersect there.
The endpoints of the base of the triangle may also be adjusted.

Note: there are many right-triangles with the orthocenter coinciding with a base endpoint, even when the base is of a fixed length, as in this figure. There is no triangle solution when the orthocenter is on the line containing the base. The triangle is degenerate when the orthocenter is on a perpendicular to the base, through a base endpoint.