FIX: remove SCSS mixed declarations (#143)

This fixes the SCSS mixed declaration errors (most noticeable while
running rspec)

scss/box-view.scss

@@ -32,19 +32,19 @@
 }
 
 .experimental-screen {
-  @include breakpoint(tablet) {
-    display: none;
-  }
   max-width: unset !important;
-  @media screen and (max-width: 488px) {
-    display: none;
-  }
   width: 100%;
   display: block;
   margin: 0;
   padding: 0;
   position: relative;
   container: content-width / inline-size;
+  @media screen and (max-width: 488px) {
+    display: none;
+  }
+  @include breakpoint(tablet) {
+    display: none;
+  }
   li {
     list-style: none;
     margin: 0;
@@ -59,6 +59,12 @@
     height: var(--d-border-radius-large);
     background-color: var(--background-color);
     z-index: 399;
+    mask: radial-gradient(
+      circle at var(--d-border-radius-large) var(--d-border-radius-large),
+      transparent var(--d-border-radius-large),
+      transparent var(--d-border-radius-large),
+      black var(--d-border-radius-large)
+    );
     -webkit-mask: radial-gradient(
       circle at var(--d-border-radius-large) var(--d-border-radius-large),
       transparent var(--d-border-radius-large),