GUACAMOLE-252: Ensure display container is explicitly sized to cover the vertical area of its container (rather than relying on the implications of display: table-cell).

guacamole/src/main/webapp/app/client/styles/display.css

@@ -46,6 +46,7 @@ div.displayOuter {
 
 div.displayMiddle {
     width: 100%;
+    height: 100%;
     display: table-cell;
     vertical-align: middle;
     text-align: center;