Modal jumps position when the viewport is resized

Created by: valerie-makes

Demo: https://jsfiddle.net/techdavid/g89ts0rs/2/

When the viewport is adjusted, the modal gets a margin applied to it, making it jump position slightly to the left (once only).

For the problem to occur, the main page content needs to overflow but not the modal. _adjustDialog() should be called when the modal opens initially so that this doesn't occur.