How about this: The modal shows up when you move the mouse out of the viewport (e.g. via the top edge), but then the modal closes immediately once the mouse cur

How about this: The modal shows up when you move the mouse out of the viewport (e.g. via the top edge), but then the modal closes immediately once the mouse cursor enters the viewport again. So, as long as (and only when) the mouse is above/outside the viewport, the modal is displayed. Also, the modal doesn't have any buttons or fields. It's just displayed for informative purposes, e.g. to inform the user about a site update, or to point out a site feature. I think that could work.