fix pause/help overlay position in scroll mode (closes #3542)

css/reveal.scss

@@ -1445,7 +1445,8 @@ $overlayHeaderPadding: 5px;
 	width: 100%;
 	height: 100%;
 	z-index: 1000;
-	background: rgba( 0, 0, 0, 0.9 );
+	background: rgba( 0, 0, 0, 0.95 );
+	backdrop-filter: blur( 6px );
 	transition: all 0.3s ease;
 }
 
@@ -1589,7 +1590,6 @@ $overlayHeaderPadding: 5px;
 		padding-bottom: 20px;
 	}
 
-
 /*********************************************
  * PLAYBACK COMPONENT
  *********************************************/
@@ -1901,6 +1901,11 @@ $notesWidthPercent: 25%;
 		display: none !important;
 	}
 
+	.overlay,
+	.pause-overlay {
+		position: fixed;
+	}
+
 	.reveal {
 		overflow: visible;
 		touch-action: manipulation;