Modal bearing (scroll) does not work when dropdown is open

Question

Please see in jsfiddle: https://jsfiddle.net/znrktLej/11/

With dropdown menu open, modal scroll does not work, gets frozen.

What am I doing wrong ?

Follows code:

.modal-body {
    max-height: calc(110vh - 210px);
    overflow-y: auto;
}

Follow the image:

Red line does not work while yellow line is open.

Browsers tested:

Does not work: Google Chorme and Opera.

Works: Firefox, Microsoft Edge, Internet Explorer