I have a modal dialog in my app which can get quite long in the y direction. This introduces a problem whereby some of the content of the dialog is hidden off the bottom of
position:fixed implies that, well, the modal window will remain fixed relative to the viewpoint. I agree with your assessment that it's appropriate in this scenario, with that in mind why don'y you add a scrollbar to the modal window itself?
If so, correct max-height and overflow properties should do the trick.