Merge branch 'add-refused-to-display-tips-to-link-preview' of https://github.com/Huxpro/reveal.js into dev

css/reveal.scss

@@ -1246,6 +1246,16 @@ body {
 		visibility: visible;
 	}
 
+	.reveal .overlay.overlay-preview.loaded .viewport-inner  {
+		position: absolute;
+		z-index: -1;
+		left: 0;
+		top: 60px;
+		width: 100%;
+		text-align: center;
+		letter-spacing: normal;
+	}
+
 	.reveal .overlay.overlay-preview.loaded .spinner {
 		opacity: 0;
 		visibility: hidden;

js/reveal.js

@@ -1603,6 +1603,9 @@
 			'<div class="spinner"></div>',
 			'<div class="viewport">',
 				'<iframe src="'+ url +'"></iframe>',
+				'<small class="viewport-inner">',
+				'This link is refused to display in a frame due to its policy',
+				'</small>',
 			'</div>'
 		].join('');