Fix viewport height on mobile browsers

gui/index.html

@@ -20,7 +20,8 @@
             #canvas {
                 /* Force Slint to occupy the entire viewport */
                 width: 100vw !important;
-                height: 100vh !important;
+                height: 100vh !important;  /* if browser doesn't support dvh */
+                height: 100dvh !important;
             }
         </style>
     </body>