Hi,
We've run into this issue whilst using the Lithium theme. See the post below:
Our users were experiencing the close 'X' button dissapearing in both IE11 and Chrome. The IE 11 issue was at 100% zoom and only affected one user but in Chrome the X would dissapear when zooming below 100%.
After investigation I've fixed it by making the following change to the CSS. So, if you run into this issue try this fix:
Edit the "Theme.css" file for the Lithium theme (and I'm assuming for any other theme where this issue is experienced) :
.theme-entry .dialog .popup-header-wrapper > .wrapper > div{ position: static; right: auto; left: auto; display: inline; /* fix for disappearing X close button */ float: none; vertical-align: middle; /*override - base*/ /*override - base*/ /*override - base*/}
.theme-entry .dialog.sub-view .popup-header-buttons,.theme-entry .dialog.sub-form .popup-header-buttons{ width: 72px; /* Fix for missing close X when browser not 100% zoom */}
Cheers,
Mike