Sliders of off screen energy sources can be on screen (but invisible)

[KatieWoe]

For phetsims/qa#483. First seen on iPad, but can be seen on other devices if you make the window very tall.
This is a very minor issue and can probably be closed without doing anything.
If you showPointerAreas, you can see and manipulate the sliders of energy sources above the current one. When the energy sources changes the sliders reset, and the sliders are otherwise invisible. Unable to find any significant issues due to this.

[chrisklus]

Nice find @KatieWoe!

This is due to the off screen system elements no longer being invisible, but opacity: 0 instead. It's caused by the changes from #305. @arouinfar seems okay to me (showPointerAreas seems pretty phet internal), what do you think?

[arouinfar]

Nice @KatieWoe!

This is due to the off screen system elements no longer being invisible, but opacity: 0 instead. It's caused by the changes from #305. @arouinfar seems okay to me (showPointerAreas seems pretty phet internal), what do you think?

Agreed @chrisklus. I think this is safe to close.