From Showit Manual
The Full Screen Widget creates a button that you can place on any page of your site that will allow users to view your site in full screen mode.
Special Note If you are viewing a site in full screen mode and click a text field, you will be brought out of full screen mode. This is not a bug but rather to prevent users from being "stuck" inside of one text box and not being able to type in other text boxes.
Check out this quote and article from Adobe for further information:
For content that is running in a browser, keyboard usage is restricted in full-screen mode. In Flash Player 9, only keyboard shortcuts that return the application to normal mode, such as pressing the Escape key, are supported. Users can’t enter text in text fields or navigate around the screen. In Flash Player 10 and later, certain non-printing keys (specifically the arrow keys, space, and Tab key) are supported. However, text input is still prohibited.