fixes

scss/_animation.scss

@@ -13,11 +13,9 @@
 
 .collapse {
   display: none;
-  visibility: hidden;
 
   &.in {
     display: block;
-    visibility: visible;
   }
   // tr&.in    { display: table-row; }
   // tbody&.in { display: table-row-group; }
@@ -29,5 +27,5 @@
   overflow: hidden;
   transition-timing-function: ease;
   transition-duration: .35s;
-  transition-property: height, visibility;
+  transition-property: height;
 }

scss/_buttons.scss

@@ -50,13 +50,12 @@
     opacity: .65;
     @include box-shadow(none);
   }
+}
 
-  a& {
-    &.disabled,
-    fieldset[disabled] & {
-      pointer-events: none; // Future-proof disabling of clicks on `<a>` elements
-    }
-  }
+// Future-proof disabling of clicks on `<a>` elements
+a.btn.disaabled,
+fieldset[disabled] a.btn {
+  pointer-events: none;
 }