Bug 1583135 - Disable scroll-anchoring at all editable elements r=emilio

This patch disables scroll-anchoring of all editable elements including editable
and scrollable elements in editing host because users don't want to scroll down
when they insert new line and new line can be in the scrolled area.

Unfortunately, the bug is not reproducible with simple testcase with
synthesized keyboard events.  Therefore, this patch does not include automated
tests.

Differential Revision: https://phabricator.services.mozilla.com/D60098

--HG--
extra : moz-landing-system : lando

layout/style/contenteditable.css

@@ -5,6 +5,14 @@
 
 @namespace url(http://www.w3.org/1999/xhtml); /* set default namespace to HTML */
 
+/* Scroll-anchoring shouldn't work in any editable and scrollable elements when
+   user inserts something.
+*/
+*|*:-moz-read-write:focus,
+*|*:root:-moz-read-write {
+  overflow-anchor: none;
+}
+
 *|*::-moz-canvas {
   cursor: text;
 }