Although this ticket may be a bit old, I encountered a similar issue while using my custom scrollbar solution and trying to drag between Sortable elements with overflow hidden. Upon adding some code to address the compatibility of Sortable with my Scrollpane, I noticed a potential oversight in the appendTo functionality.
The current implementation of appendTo only appends the helper to the target if it is not already present in the DOM. This explains why the clone option works for some cases but not others (which I won't delve into here). The crucial step to resolve this was to incorporate the following code towards the end of the _mouseStart function within the widget:
if (!this.helper.parent().is(this.appendTo)) {
this.helper.detach().appendTo(this.appendTo);
// update position
this.offset.parent = this._getParentOffset();
}
It's worth noting that this.appendTo is defined earlier in the function as follows:
this.appendTo = $( o.appendTo !== "parent" ?
o.appendTo :
this.currentItem.parent() );
By implementing this fix, I specified an appendTo targeting the div containing both Sortable elements, effectively resolving the overflow problem.
You can find the complete solution, including other fixes, in the scrollsortable JS file for jQuery-UI-ScrollPane at: https://github.com/borgboyone/jQuery-UI-ScrollPane.
Cheers!