Bug 1645794 - Set -moz-appearance:none on textareas, to avoid unreliable sizing when setting/removing background. r=emilio

See Layout bug 1645773 for details of the bug being wallpapered here.

Differential Revision: https://phabricator.services.mozilla.com/D79684

dom/html/reftests/autofocus/style.css

@@ -1,5 +1,8 @@
 :focus { background-color: green; }
 
+/* workaround for https://bugzilla.mozilla.org/show_bug.cgi?id=1645773 */
+textarea { -moz-appearance: none; }
+
 /**
  * autofocus is considered like a keyboard focus and .focus() isn't.
  * We might change that with bug 620056 but for these tests, we don't really care.