JQuery/GSAP - Moving multiple elements parallel to

What I want to do is this: I want to generate boxes on the Y or X axis of the page (the bottom and left border) and make them move parallel to each other until they've gone off-screen, where I want to delete them. This is what I've done so far:

Codepen.

function generate(){
  var $element = $("<div>", {class: "box"});
  //Generate random x and y coordinates
  var posy = (Math.random() * ($('body').height() - $element.width())).toFixed();
  var posx = (Math.random() * ($('body').width() - $element.height())).toFixed();
  //Place the box on the x or y axis
  var rand = Math.floor(Math.random()*2);
  if(rand==0)
  {
    posx=0;
  }else{
    posy=0;
  }

  $element.css(
  {'position':'absolute',
  'right':posx+'px',
  'bottom':posy+'px'});
  $("body").append($element);

  //Move box diagonally, all the boxes paths are parallel
    TweenLite.to($element, 2, {top:0, right:0, ease:Power2.easeOut});

  //Delete boxes offscreen
  var $boxes=$(".box");
  /*$boxes.each(function(){
    if($(this).position().left >1300){
      $(this).remove();
    }
  });*/
}

My issues are: how do I exactly find the right point where to animate the box, in a way to move all the boxes parallel to each other? And also, is there a way without using external plugins to find if an element has gone out of the bounds of the page? Thanks for any answer!

EDIT: Here's a drawing to explain myself better. Sorry for my terrible Paint skills!

Now all the boxes converge to the same top:0 & right:0 point, but I'd like to make them go parallel to each other, and so, each one of them should have a different top/right point to go. I'm guessing my issue would be to calculate it, but how do I do exactly? And also this meddles with the off-screen issue I take.

Is this the kind of effect you are trying to create?

JavaScript:

function generate(){
  var windowHeight = $(window).height();
  var windowWidth = $(window).width();
  var $element = $('<div>', {class: 'box'});
  var initX = parseInt(Math.random() * windowWidth) - (windowWidth * 0.5);
  var destX = parseInt(initX) + $(window).height();

  $('body').append($element);
  TweenLite.fromTo($element, 2, {x:initX, y:0}, {x:destX, y:-windowHeight, ease:Power2.easeOut, onComplete: onCompleteAnimation, onCompleteParams: [$element]});
}

function onCompleteAnimation($element) {
  $element.remove();
}

You will notice that I am not animating top and left (or right) properties with position: absolute; that you were previously doing. Instead, I am animating x and y which are essentially translate(x, y) values. This makes the animation look smooth: Link.

Hope this helps.