I understand the movement of a fixed rod. But in the layout nothing is visually changing except the point I'm moving, while the numbers all change.

I see now that C' is for the lid closed, so I better understand how it all works. It's just very difficult for me to do what I need to do with the setup. Wile moving point B around there's no way to keep BC1 at a fixed length - am I guessing that as long as it's 1.0 then it represents 100% length? However, if so, I don't know why it doesn't go red when moving above 1.0

What I was describing was having only the single line representing the spring and to see it closed you'd change the angle of A. In other words, it would be more like a simulation of the real product rather than plotting simultaneously both open and closed.

While it's true that the top mounting point will be set in (or upward) of the edge of the top, the location along the edge of the top, represented by the distance from A to C also needs to change. Having a slider for that or simply being able to move the point would be very useful. The lines currently represented by AC and AC' could be replaced by ASomethingelse and ASomethingelse' with the current C points sitting on that line.