Add FIXME for virtual keyboard button on touch

The way we decide whether to show the keyboard button or not is not
ideal, let's add a FIXME for that.

app/styles/base.css

@@ -554,6 +554,11 @@ html {
   display: none;
 }
 @media not (any-pointer: coarse) {
+  /* FIXME: The button for the virtual keyboard is the only button in this
+            group of "mobile buttons". It is bad to assume that no touch
+            devices have physical keyboards available. Hopefully we can get
+            a media query for this:
+            https://github.com/w3c/csswg-drafts/issues/3871 */
   :root.noVNC_connected #noVNC_mobile_buttons {
     display: none;
   }