jquery ui sortable scroll=true卻無法滾動的解決辦法

用jquery ui 做sortable功能時經常碰到設置了scroll=true，滾動條卻無法滾動的情況。
核心的代碼是

_mouseDrag: function(event) {
		var i, item, itemElement, intersection,
			o = this.options,
			scrolled = false;

		//Compute the helpers position
		this.position = this._generatePosition(event);
		this.positionAbs = this._convertPositionTo("absolute");

		if (!this.lastPositionAbs) {
			this.lastPositionAbs = this.positionAbs;
		}

		//Do scrolling
		if(this.options.scroll) {
			if(this.scrollParent[0] !== document && this.scrollParent[0].tagName !== "HTML") {

				if((this.overflowOffset.top + this.scrollParent[0].offsetHeight) - event.pageY < o.scrollSensitivity) {
					this.scrollParent[0].scrollTop = scrolled = this.scrollParent[0].scrollTop + o.scrollSpeed;
				} else if(event.pageY - this.overflowOffset.top < o.scrollSensitivity) {
					this.scrollParent[0].scrollTop = scrolled = this.scrollParent[0].scrollTop - o.scrollSpeed;
				}

				if((this.overflowOffset.left + this.scrollParent[0].offsetWidth) - event.pageX < o.scrollSensitivity) {
					this.scrollParent[0].scrollLeft = scrolled = this.scrollParent[0].scrollLeft + o.scrollSpeed;
				} else if(event.pageX - this.overflowOffset.left < o.scrollSensitivity) {
					this.scrollParent[0].scrollLeft = scrolled = this.scrollParent[0].scrollLeft - o.scrollSpeed;
				}

			} else {

				if(event.pageY - $(document).scrollTop() < o.scrollSensitivity) {
					scrolled = $(document).scrollTop($(document).scrollTop() - o.scrollSpeed);
				} else if($(window).height() - (event.pageY - $(document).scrollTop()) < o.scrollSensitivity) {
					scrolled = $(document).scrollTop($(document).scrollTop() + o.scrollSpeed);
				}

				if(event.pageX - $(document).scrollLeft() < o.scrollSensitivity) {
					scrolled = $(document).scrollLeft($(document).scrollLeft() - o.scrollSpeed);
				} else if($(window).width() - (event.pageX - $(document).scrollLeft()) < o.scrollSensitivity) {
					scrolled = $(document).scrollLeft($(document).scrollLeft() + o.scrollSpeed);
				}

			}

			if(scrolled !== false && $.ui.ddmanager && !o.dropBehaviour) {
				$.ui.ddmanager.prepareOffsets(this, event);
			}
		}

		//Regenerate the absolute position used for position checks
		this.positionAbs = this._convertPositionTo("absolute");

		//Set the helper position
		if(!this.options.axis || this.options.axis !== "y") {
			this.helper[0].style.left = this.position.left+"px";
		}
		if(!this.options.axis || this.options.axis !== "x") {
			this.helper[0].style.top = this.position.top+"px";
		}

		//Rearrange
		for (i = this.items.length - 1; i >= 0; i--) {

			//Cache variables and intersection, continue if no intersection
			item = this.items[i];
			itemElement = item.item[0];
			intersection = this._intersectsWithPointer(item);
			if (!intersection) {
				continue;
			}

			// Only put the placeholder inside the current Container, skip all
			// items form other containers. This works because when moving
			// an item from one container to another the
			// currentContainer is switched before the placeholder is moved.
			//
			// Without this moving items in "sub-sortables" can cause the placeholder to jitter
			// beetween the outer and inner container.
			if (item.instance !== this.currentContainer) {
				continue;
			}

			// cannot intersect with itself
			// no useless actions that have been done before
			// no action if the item moved is the parent of the item checked
			if (itemElement !== this.currentItem[0] &&
				this.placeholder[intersection === 1 ? "next" : "prev"]()[0] !== itemElement &&
				!$.contains(this.placeholder[0], itemElement) &&
				(this.options.type === "semi-dynamic" ? !$.contains(this.element[0], itemElement) : true)
			) {

				this.direction = intersection === 1 ? "down" : "up";

				if (this.options.tolerance === "pointer" || this._intersectsWithSides(item)) {
					this._rearrange(event, item);
				} else {
					break;
				}

				this._trigger("change", event, this._uiHash());
				break;
			}
		}

		//Post events to containers
		this._contactContainers(event);

		//Interconnect with droppables
		if($.ui.ddmanager) {
			$.ui.ddmanager.drag(this, event);
		}

		//Call callbacks
		this._trigger("sort", event, this._uiHash());

		this.lastPositionAbs = this.positionAbs;
		return false;

	},

若 if((this.overflowOffset.top + this.scrollParent[0].offsetHeight) - event.pageY < o.scrollSensitivity) {...}

這裏的 this.scrollParent[0].offsetHeight返回的是undefined，就會導致無法scroll的問題。由於我用了jquery mobile 的框架。會默認給body加上jm的class，例如overflow:hidden，會導致無法獲取offsetHeight.解決辦法，就是將 this.scrollParent[0]的所有會影響到獲取offsetHeight的class都刪掉，就可以解決drag滾動條無法滾動的問題了。