💄 Use of border-box box sizing

This makes calculating the size of the UI components easier, especially those which have padding, margin or border.

src/ui/components/modal.scss

@@ -67,7 +67,7 @@ $modal-max-height: 55vh;
             border-radius: $border-radius;
 
             &__placeholder {
-                width: calc(100% + (2 * $border-normal-width));
+                width: 100%;
                 height: 100%;
             }