v8.1.24
I found a bug where, if a popup that is wider than the viewport it's opened in (for example a mobile screen) and constrain to viewport is checked, then the popup will automatically resize, but its content won't. Scrollbars appear to view the rest of the contents of the popup. If you resize the popup however, even just slightly, the popup properly then sizes the content to the size of the popup and avoids scrollbars.
It seems that the automatic resize event isn't firing whatever event is supposed to resize the popup content as well.