Some components cause problems with other components or "external" elements.
When you put popovers on a page with modals (or image modals), the modals break.
Modals are not fully compatible with the vector skin. Therefore they are missing the backdrop. Also, you might notice a slight "wobble" in vector's header every time the modal pops up.
Some user experience broken html output when trying to use modals as the term in definition lists. Using the html equivalent causes the same problem.
Sometimes, not only in vector skin, modal backdrops (the greying out of the background) tend to stack the wrong way. In these instances the z-index of the modal and the backdrop are ignored and the backdrop overlays the modal and everything else. You cannot even close the modal and have to reload the page.
When this happens with your installation you have to disable backdrops
altogether. Please add the following to your MediaWiki:Common.css
or
anywhere else where css is processed:
.modal-backdrop {
display: none;
}
Oftentimes when you have problems with the backdrop, your z-index calculation is off. Then it can also happen, that your Navbar overlaps the modal. In that case simply "push" the modal further down. For example:
.modal {
top: 60px;
}