Out of curiosity, I've looked into this. You could theoretically get the top offset of an element with
offsetTop and the available height of the window with
window.screen.availHeight and hide the object if the vertical offset is larger than 50% of the available window height.
This method kind of works with a browser and to some extent with ADE4, but I got weird results with other ePub3 apps.
I.e., this method is probably not robust enough for practical use.
I've attached a very simple proof-of-concept test file that you can play with.
(Uncomment
//myDiv.parentNode.removeChild(myDiv); to actually delete the div.)