Position fixed for blockOverlay only in full mode? Why?

We are using blockUI for displaying multiple dialogs (that is an overlay window which might display additional sub dialogs).

Since there is no way of having multiple blockUI instances in "full" mode (right?), we use element blocking:

$('body').append('<div id="overlay"></div>');        
$('#overlay').block({
  message: '<div class="container"></div>',
  css: css
});

Now of course the problem is, that the background blocking layer overrides the CSS settings in line 270 of blockui:

That means it's impossible to cover the full screen, because position is always absolute, never fixed (if not in full mode).

Why is it so important not to use position fixed for element blocking? Is there any reason for this I might have overlooked?

What happens if you simply remove line 270? It's possible to set CSS properties for the message and the block layer anyways.

malsup / blockui