restore mixin

src/client/styles/scss/_mixins.scss

@@ -72,8 +72,8 @@
         .hackmd-error,
         #iframe-hackmd-container > iframe {
           width: 100%;
-          height: calc(90vh - #{$header-plus-footer});
-          min-height: calc(90vh - #{$header-plus-footer}); // for IE11
+          height: calc(100vh - #{$header-plus-footer});
+          min-height: calc(100vh - #{$header-plus-footer}); // for IE11
         }
       }
     }