SolveMyMaths Action 4!

Topic:
Area, Square
Creation of this applet was inspired by a tweet from Ed Southall. Feel free to move any one (or more) of the white points. You can adjust the sizes of the squares using the sliders. How does this applet informally illustrate that the areas of the 2 green triangles are equal? How can you formally prove what this applet informally illustrates?