Dropdown (select) filtering in dxtmlx treegrid

Dear dhtmlx support team,

We have a problem for treegrid select box filtering. Basically we want to refresh the content of the second filter based on selection of the first filter.
As you can see in the attached image; after the city istanbul is selected, we want the treegrid show only districts of istanbul in the districts(area) dropdown (select box).
However, this not happening, instead treegrid keep showing all the areas/districts in the districts dropdown regardless of selected city.
Please note filtrations works fine, only problem is values in the dropdown are not updated based on other filters. Any ideas how we can fix this?

Can be done by code modification.
In dhtmlxtreegrid_filter.js you can locate

var col=this._f_rowsBuffer||this._h2;

and replace it with

var col = this._h2;

now filter list will show options only for records which is visible in the treegrid.