Edit: As of the last update, this is no longer required. Only remaining problem is that it is displayed below the sticky headers which requires just this CSS to fix:
#multiple-windows .multiple-windows__document .multiple-windows__close-pane-container {
z-index: 5;
}
To install add this to your Custom CSS:
/* Always visible close button
* by hannesfrank
*/
#multiple-windows .multiple-windows__document #multiple-windows__close-pane {
/* assumes position: relative of parent multiple-windows__document */
position: sticky;
top: 5px;
right: 5px;
z-index: 2; /* display above the sticky head */
}
#multiple-windows .multiple-windows__document #multiple-windows__close-pane i.icon.close {
float: right;
width: 15px;
height: 15px;
font-size: 12px;
box-shadow: 0px 0px 1px 0px grey;
background: white;
vertical-align: bottom;
}
#multiple-windows .multiple-windows__document #multiple-windows__close-pane i.icon.close::before {
position: relative;
top: 1px;
}