I've often noticed this when usin iPad but not iPhone. Seems to occur if you touch the screen while the page is loading.
Little tip for you - turning the iPad through 90 degrees to change between portrait and landscape modes then turning back to the original orientation seems to fix it every time, avoiding the need to do a page refresh. This is especially handy if you've written a post and upon trying to submit it find the lower half of the page including the post button is missing. A page refresh will blank your unsubmitted post whereas swapping between portrait and landscape modes and back is the work of moments.