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I did check your form in my iPhone device and it seems to work as expected. While I sign in the widget, it does not move the screen.
Please be noted that the height of your "Smooth Signature" widget is too small. That could be the reason why the screen moves. I would suggest you to please increase the height of the widget and that should most likely solve your problem. I would also suggest you to make your form mobile responsive as well. Please check the screenshot below:
Please take a look at the following cloned form and see if this works as expected: http://form.jotformpro.com/form/52082264835961?
Feel free to clone this form for a closer look. The following guide should help you in form cloning: http://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL
Hope this helps.
Do get back to us if you have any questions.