Hover styling in IE11 + Edge not working when pointer over textarea

Problem/Motivation

Discovered in #3078524: Focus styling of WYSIWYG field inputs should be consistent with other inputs. CKEditor provides a black outline on hover. In IE + Edge, there is no hover styling when the pointer is over the textarea (or more specifically, the iframe). It was posited that this is happening because these browsers do not consider the iframe part of the document, and do not extend styling to it.

Proposed resolution

Most likely a JavaScript approach. The CKEditor container could have a mouseover listener that would trigger the addition of a class that provides hover styling.

Remaining tasks

Try out some approaches, pick one that seems good, implement it.

User interface changes

Hovering over CKEditor in IE11 + Edge will have behavior consistent with other browsers.

API changes

Data model changes

Release notes snippet