There seems to be a minor bug. When I switch tabs and come back, sometimes the spring is moving. Some times a small amount, and other times it appears to be streched to the max, and extending off the top and bottom of the screen, until it calms down.

Safari, Mac.

I just wanted to write about a similar observation when using it in FireFox on Linux:

When wiggle the spring, keep the mouse inside the white area until it is at rest, press CTRL+u to see the source code, move the mouse to close the source code tab and close it - for some magical reason the spring is moving again for a little bit.