The reason that nothing happens in my example seems to be that the receiver container's height is smaller than the helper's height. In the jquery.ui.sortable.js code I find this in the
if ( this.options.tolerance === "pointer" || this.options.forcePointerForContainers ||
(this.options.tolerance !== "pointer" &&
this.helperProportions[this.floating ? "width" : "height"] > item[this.floating ? "width" : "height"])
} else ...
which specifies exactly that behavior (for the case that the
tolerance option is set to
intersect which is the default): if the helper height is larger than the receiver container's height, return true only if the mouse pointer is over the container.
I don't know why this is there. (Maybe it has some use while sorting within one container.)