Square + Rhombus = Constant Surprise! (GoGeometry Action 71)

Creation of this applet was inspired by a tweet from Antonio Gutierrez (GoGeometry). You can adjust the size of the blue angle (soon to appear) using the blue slider. You can also move any of the 2 LARGE WHITE POINTS anywhere you'd like. What is the measure of the pink angle? How can we formally prove this is ALWAYS TRUE (regardless of what the original rhombus looks like?)

Quick (Silent) Demo