It looks like that the 'grid' option in the constructor of Draggable is relatively bound to the original coordinates of the element being dragged - so simply put, if you have three draggable divs with their top set respectively to 100, 200, 254 pixels relative to their parent:

<div class="parent-div" style="position: relative;">
    <div id="div1" class="draggable" style="top: 100px; position: absolute;"></div>
    <div id="div2" class="draggable" style="top: 200px; position: absolute;"></div>
    <div id="div3" class="draggable" style="top: 254px; position: absolute;"></div>
</div>

Adn all of them are getting enabled for dragging with 'grid' set to [1, 100]:

draggables = $('.draggable');
$.each(draggables, function(index, elem) {
    $(elem).draggable({
                       containment: $('#parent-div'),
                       opacity: 0.7,
                       revert: 'invalid',
                       revertDuration: 300,
                       grid: [1, 100],
                       refreshPositions: true
    });
});

Problem here is that as soon as you drag div3, say, down, it's top is increased by 100, moving it to 354px instead of being increased by just mere 46px (254 + 46 = 300), which would get it to the next stop in the grid - 300px, if we are looking at the parent-div as a point of reference and "grid holder".

I had a look at the draggable sources and it seem to be an in-built flaw - they just do all the calculations relative to the original position of the draggable element.

I would like to avoid monkey-patching the code of draggable library and what I am really looking for here is the way how to make the Draggable calculate the grid positions relative to containing parent. However if monkey-patching is unavoidable, I guess I'll have to live with it.

Thanks!