u_mouse causing flashing in shader

[CarlyRaeJepsenStan]

Hey everyone, I have the following shader:
https://glslsandbox.com/e#81208.1

It responds to mouse movement smoothly.

However, the same shader using glslsandbox:
https://shaders-threejs.glitch.me

Moving around the cursor causes the screen to flash blue, a markedly different effect from the expected behavior. Any ideas how I can fix this, asides from removing the u_mouse variable completely?