Edit in JSFiddle

// Draggable Inner by Zzbaivong (https://github.com/baivong)

function draggableInner(drag, container) {

  function mouseUp(e) {
    if (e.which !== 3) return;

    window.removeEventListener('mousemove', mouseMove, true);
    drag.style.cursor = 'default';
  }

  function mouseDown(e) {
    if (e.which !== 3) return;

    posInDrag = {
      x: (e.pageX - drag.offsetLeft),
      y: (e.pageY - drag.offsetTop)
    };
    window.addEventListener('mousemove', mouseMove, true);
    drag.style.cursor = 'move';
  }

  function mouseMove(e) {
    if (e.which !== 3) return;

    posInPage = {
      x: e.pageX,
      y: e.pageY
    };

    current = {
      x: (posInPage.x - posInDrag.x),
      y: (posInPage.y - posInDrag.y)
    };

    if (current.x < min.x) current.x = min.x;
    if (current.x > max.x) current.x = max.x;

    if (current.y < min.y) current.y = min.y;
    if (current.y > max.y) current.y = max.y;

    drag.style.left = current.x + 'px';
    drag.style.top = current.y + 'px';
  }

  var posInDrag = posInPage = containerSize = dragSize = min = max = current = {};

  if (!container) {
    container = document.body;
  }

  containerSize = {
    w: container.clientWidth,
    h: container.clientHeight
  };
  dragSize = {
    w: drag.clientWidth,
    h: drag.clientHeight
  };

  min = {
    x: container.offsetLeft,
    y: container.offsetTop
  };
  max = {
    x: containerSize.w - dragSize.w + min.x,
    y: containerSize.h - dragSize.h + min.y
  };

  drag.addEventListener('mousedown', mouseDown, false);
  window.addEventListener('mouseup', mouseUp, false);
  drag.oncontextmenu = function(e) {
    e.preventDefault();
  };

}

draggableInner(document.getElementById('drag'), document.getElementById('container'));

<div id="container">
  <div id="drag"></div>
</div>

html,
body {
  margin: 0;
  padding: 0;
}

#container {
  width: 500px;
  height: 500px;
  background: #39464E;
  margin: 10px;
  overflow: auto;
}

#drag {
  position: absolute;
  width: 400px;
  height: 300px;
  background: #FF9800;
  opacity: 0.7;
}