Each call to $el.selectable('refresh') is slower than the previous, starting from 200ms and going to more than 10 seconds. (for hundreds of selectees)

The problem comes from the line:

that._addClass( that.selectees, "ui-selectee" );

I have seen an issue for v1.12.0 and upgraded to v1.12.1, but it did not solve the problem.

Replacing the line above by:

that.selectees.addClass("ui-selectee");

solves the performance issue.

When dragging across multiple sortable elements during one drag event, the ui.sender should update to the most recent sortable element dragged through. It currently remains set to the original container.

Trying to select another month by activating selectOtherMonths does not work.
If you select a date of the previous month, it selects that date of the current month. If you select 31 of the last month, the first day of the next month is selected instead.

After looking at the code and testing, I have found a fix by adding the following after line 867 in _selectDay:

if (month != inst.selectedMonth) {
  if (month > inst.selectedMonth && year < inst.selectedYear)
    this._adjustDate('#datepicker', -1, 'M');
  else if (month < inst.selectedMonth && year > inst.selectedYear)
    this._adjustDate('#datepicker', +1, 'M');
  else if (month > inst.selectedMonth)
    this._adjustDate('#datepicker', +1, 'M');
  else if (month < inst.selectedMonth)
    this._adjustDate('#datepicker', -1, 'M');
}