Bug 1766093 - Prevent text selection inside the video controls. r=Gijs

It seems we don't allow it generally...

I couldn't reproduce the bad effect on standalone pages (so there's
something about the YouTube markup or how they setup the event listeners
or some other interaction going on), but:

 * This fixes the issue and is extremely simple.
 * It doesn't prevent users from selecting around videos and pasting
   them into rich text editors.
 * We already apply user-select: none to top-level video documents.

So over-all it should be pretty safe.

Differential Revision: https://phabricator.services.mozilla.com/D147301

toolkit/themes/shared/media/videocontrols.css

@@ -12,6 +12,10 @@
   overflow: hidden;
 
   direction: ltr;
+  /* Prevent selection from interacting weirdly with the page,
+   * see bug 1766093. Our text selection story with shadow dom should be
+   * better, see bug 1590379 */
+  user-select: none;
   /* Prevent unwanted style inheritance. See bug 554717. */
   text-align: left;
   list-style-image: none !important;