Edit in JSFiddle

function isDraggableHelperInsideDroppableContainer(event, $droppableContainer)
{
    var cTop = $droppableContainer.offset().top;
    var cLeft = $droppableContainer.offset().left;
    var cBottom = cTop + $droppableContainer.height();
    var cRight = cLeft + $droppableContainer.width();

    if (event.pageY >= cTop && event.pageY <= cBottom && event.pageX >= cLeft && event.pageX <= cRight)
    {
        return true;
    }
    else
    {
        return false;
    }
}

$('.item').droppable({
    tolerance: "intersect",
    activeClass: "ui-state-hover",
    accept: "#draggable",
    drop: function (event, ui)
    {
        var dropItem = $(this);
        
        if( ! isDraggableHelperInsideDroppableContainer(event, dropItem.closest(".container")) )
        {
            console.log("Drop event returning false to cancel drop because it occurred outside the droppable container!");
            return false;
        }
        
        alert("Successfully dropped draggable onto element " + dropItem.text() );
    },

    over: function (event, ui)
    {
        var dropItem = $(this);

        if (isDraggableHelperInsideDroppableContainer(event, dropItem.closest(".container")))
        {
            dropItem.addClass("dropTargetValid");
        }
    },
    
    out: function (event, ui)
    {
        var dropItem = $(this);
        dropItem.removeClass("dropTargetValid");
    },
    
    deactivate: function (event, ui)
    {
        var dropItem = $(this);
        dropItem.removeClass("dropTargetValid");
    }
});

$('#draggable').draggable();

<div class="container">
    <div class="item">Test1</div>
    <div class="item">Test2</div>
    <div class="item">Test3</div>
    <div class="item">Test4</div>
    <div class="item">Test5</div>
    <div class="item">Test6</div>
    <div class="item">Test7</div>
    <div class="item">Test8</div>
    <div class="item">Test9</div>
    <div class="item">Test10</div>
    <div class="item">Test11</div>
    <div class="item">Test12</div>
    <div class="item">Test13</div>
    <div class="item">Test14</div>
    <div class="item">Test15</div>
    <div class="item">Test16</div>
    <div class="item">Test17</div>
    <div class="item">Test18</div>
    <div class="item">Test19</div>
    <div class="item">Test20</div>
    <div class="item">Test21</div>
    <div class="item">Test22</div>
    <div class="item">Test23</div>
    <div class="item">Test24</div>
    <div class="item">Test25</div>
    <div class="item">Test26</div>
    <div class="item">Test27</div>
    <div class="item">Test28</div>
</div>

<div id="draggable">Drag Me!</div>

.container {
    position: absolute;
    top: 50px;
    left: 50px;
    height:100px;
    width:200px;
    overflow-x:hidden;
    overflow-y:auto;
    border:1px solid #000000;
}

.item {
    border:1px solid #ff0000;
    height:30px;
    width: 500px;
}

#draggable {
    border:2px solid #00ff00;
    height:20px;
    width:100px;    
}

.ui-state-hover {
    background-color:#eeeeee;
}

.dropTargetValid {
    background-color: green !important;
}