Index: src/wp-admin/css/forms.css
===================================================================
--- src/wp-admin/css/forms.css	(revision 46370)
+++ src/wp-admin/css/forms.css	(working copy)
@@ -315,6 +315,7 @@
 	border-radius: 3px;
 	padding: 3px 24px 3px 8px;
 	min-height: 28px;
+	max-width: 25rem;
 	vertical-align: middle;
 	-webkit-appearance: none;
 	/* The SVG is arrow-down-alt2 from Dashicons. */