Because a picture is worth a thousand words:
(Using Firefox for Ubuntu 50.1.0 on Ubuntu 16.04)
A select box appears to be overflowing its .postbox > .inside > fieldset > fieldgroup parent in the two-column layout.
The particular select box in question is on a BuddyPress Groups > Edit page with bbPress forums integration (i.e., the “Discussion Forum” meta box). However, neither of these plugins appears to have designated any additional CSS for handling the declaration in /wp-admin/css/forms.css L. 320:
.wp-admin select {
padding: 2px;
line-height: 28px;
height: 28px;
}
During browser inspection, specifying width: 100%; seemed to fix it, but not max-width: 100%; (which is declared for .meta-box-sortables select on L: 332).
It’s probably best if you post a question on the buddpress support site. If they think you should add it to their trac, then they’ll give you the link.
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