Program works differently in full screen and editor mode

[f_m_fmbg]

1. What browsers show this problem?
   Firefox
2. Please share an example project (if possible).
   how it looks on the website, how it looks in the editor
3. Describes the steps to reproduce this issue.
   Run the project. When it draws the underpart switch between normal and full screen/presentation mode
4. What does Snap! currently do?
   full screen/presentation mode: it draws nothing or only a part
   
   
   
   
   editor mode: it draws the entire picture
   
   
   
   website mode: it draws parts (after switching back from presentation mode)
   
   
5. What should Snap! do instead?
   draw the entire picture every time

[dardoro]

Try this to see real value of sprite position

You can see that "x position", sometimes, has no exact value of 1.
Snap stores sprite position "scaled" by "stage.scale" (zoom).
So your calculation for negative value (y) and non integer exponent (y) returns NaN (for unsupported imaginary number).
You can round "x position" to calculate y*x.

Also SpriteMorph.prototype.xPosition should be modified to preserve exact sprite position.