[a11y]: Modal: The 'fade' effect on dialogs with overflow body text creates some odd behaviours

[tay1orjones]

Originally outlined in #12727:

The Carbon guidance on Overflow is that "The content should visibly fade at the end of the modal body area to indicate there is additional content out of view."

However, scroll bars are already indicating additional content.

There are negative results of this 'fade area':

1. Especially with forms, the fade affect doesn't really work, so the bottom part of the dialog is effectively a bunch of padding
   
2. The fading area has been implemented in a way that makes the browser think it is not within the modal, so if a user clicks inside the dialog in this space, the dialog is dismissed, as if the user had clicked outside it (which, as per issue 3, should only be happening on the passive -- which should never have enough text to need scrolling IMO) Sev 2
   Here is a shot of a dialog with scrolling added to the body, so you can see the area in the dialog affected by this