Plugin to set a class on the first visible element of a class. Options are:

 
watchClass: false, // what class are we looking for ( needs to be set )
activeClass: 'topElement', // class to add to found el.
callback: false, // function to call on change, is passed (null, current hilighted el, el about to be hilighted)
showRatio: 1, // what percent of us can be off-screen and still count as "visible" ? 0 = we are not "visible" once ANY part is offscreen. 
            // .5 = half of us can be off screen and we still count as "Visible"
            // 1 = as long as one pixel is onscreen, we are "visible"
padValue: 0 // padding in px you may want to add to visiblity calculations  
First Div

This page is using: jQuery().getFirstVisible({ watchClass: 'activity', showRatio: 0.75 });

This is the second div
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity
Some activity