IEF popup breaks Tagify entity reference widget

Problem/Motivation

The IEF Complex Widget Dialog has CSS issues when used in combination with the Tagify → entity reference widget, like in the images below.

Incorrect display

Correct display

Proposed resolution

The problem appears to be caused by line 18 in the CSS code of ief_popup.css:

.ief-popup-wrapper div,
.ief-popup-wrapper fieldset {
    position: relative;
}

In my case, changing line 18 to something more specific, like

.ief-popup-wrapper div.ui-widget-content

solved the problem.