fixes #15830: properly reset border-radius on list items in collapsing panel groups

dist/css/bootstrap.css

@@ -5504,6 +5504,10 @@ a.list-group-item-danger.active:focus {
   border-bottom-right-radius: 3px;
   border-bottom-left-radius: 3px;
 }
+.panel > .panel-heading + .panel-collapse > .list-group .list-group-item:first-child {
+  border-top-left-radius: 0;
+  border-top-right-radius: 0;
+}
 .panel-heading + .list-group .list-group-item:first-child {
   border-top-width: 0;
 }

less/panels.less

@@ -76,6 +76,7 @@
         .border-top-radius((@panel-border-radius - 1));
       }
     }
+
     // Add border bottom radius for last one
     &:last-child {
       .list-group-item:last-child {
@@ -84,6 +85,11 @@
       }
     }
   }
+  > .panel-heading + .panel-collapse > .list-group {
+    .list-group-item:first-child {
+      .border-top-radius(0);
+    }
+  }
 }
 // Collapse space between when there's no additional content.
 .panel-heading + .list-group {