The domain parameters doesn't change if "c2tnb431r1" is chosen (Curve type F(2^m), Curve size Large).
When a lagre curve size is activated, it is possible to insert zero or a negative number for Alice' and Bob's secret. This causes program errors.
The information pop-ups are very annoying. I suggest to add a description box in the excess space in the right instead.
The button "Choose random curve" changes its position while changing the curve size.
The value of G doesn't change when changing the curve size from large to small.
An open bracket is missing when large numbers are used (Public patrameters box --> Generator G).