When editing a product in Fancy Product Builder, I can move an element around using the mouse just fine. But when I enter a number in the X or Y fields, the element disappears completely, and I cannot get it back. Using FPD 2.1.0 and latest WP, WooCommerce and Chrome.
I've had a similar problem, not sure if it's related though.
It looks like it's in the way the data is updated when typing. If you write "250", the system will in fact position your element at "25", the last digit is not updated. I use arrows up/down to force the preview to update to the right position. You have to pay attention though because sometime it seems the preview is off by 1px.
Super frustrating... After you enter the desired number, use the tab key to move to the next box.