I'm having a debate with colleagues about the way our modal windows work on our website. The two options are, fixed positioning - so that when the window pops up, it's stuck in the middle of the screen, no matter where you scroll.
The other option is absolute positioning - so that the window pops up to the middle of the screen and then stays at that point, if you scroll up or down it will go up and down with the rest of the page.
Pros for fixed
Window is always visible - the user has to interact with it, and cannot "loose" the window
Cons for fixed
Mobile view especially - but affects desktop too - If a mobile user wants to zoom in (not usually needed - as our site is fully responsive, so text/images should always be the appropriate size) the fixed popup will end up half off the page with no way of viewing the rest of it unless you fully zoom out again, making zooming in basically unusable.
If the modal window is too large to fix on the page - then the modal window needs to be scrollable, which is not great for usability IMO, plus on some mobile devices this function does not work very well and can make it harder to use.
Pros for Absolute
Scrolling is always the same - regardless of the modal height. Zooming is always the same - nothing unexpected occurs when zooming.
Cons for Absolute
It's relatively easy to loose the popup from view if it's small and the user is scrolling when the popup is shown.
If the modal window is very large it might end up showing the popup longer than the html page - very unlikely occurrence but I guess it's a possibility.
The absolute positioned window popups up at the current point that the user has scrolled to - so it never pops up off screen.
The modal window is built so that clicking on the darkened background will close the modal window, so regardless of positioning - the user can always close the window if they get lost.
I'm in favour of the absolute positioning as IMO the usability is far greater - the sticking point for this debate to keep coming up is the ability to loose the window when scrolling - however I think that issue is far less of a problem than the usability problems that occur with fixed positioning.
I can add some screenshots if needed, but wasn't sure they would actually help show off the issue.