Avoid ns-error-failure and client crash. close #6857 and close #6710

bigbluebutton-html5/imports/ui/components/media/styles.scss

@@ -35,7 +35,12 @@
 }
 
 .hideOverlay {
-  display: none;
+  visibility: hidden;
+  position: absolute;
+  overflow: hidden;
+  clip: rect(0 0 0 0);
+  height: 1px; width: 1px;
+  margin: -1px; padding: 0; border: 0;
 }
 
 .floatingOverlay {