Nested button groups with only one child now have rounded corners (Iss.#15270)

dist/css/bootstrap.css

@@ -3577,12 +3577,12 @@ tbody.collapse.in {
 .btn-group > .btn-group:not(:first-child):not(:last-child) > .btn {
   border-radius: 0;
 }
-.btn-group > .btn-group:first-child > .btn:last-child,
-.btn-group > .btn-group:first-child > .dropdown-toggle {
+.btn-group > .btn-group:first-child:not(:last-child) > .btn:last-child,
+.btn-group > .btn-group:first-child:not(:last-child) > .dropdown-toggle {
   border-top-right-radius: 0;
   border-bottom-right-radius: 0;
 }
-.btn-group > .btn-group:last-child > .btn:first-child {
+.btn-group > .btn-group:last-child:not(:first-child) > .btn:first-child {
   border-top-left-radius: 0;
   border-bottom-left-radius: 0;
 }

docs/dist/css/bootstrap.css

@@ -3577,12 +3577,12 @@ tbody.collapse.in {
 .btn-group > .btn-group:not(:first-child):not(:last-child) > .btn {
   border-radius: 0;
 }
-.btn-group > .btn-group:first-child > .btn:last-child,
-.btn-group > .btn-group:first-child > .dropdown-toggle {
+.btn-group > .btn-group:first-child:not(:last-child) > .btn:last-child,
+.btn-group > .btn-group:first-child:not(:last-child) > .dropdown-toggle {
   border-top-right-radius: 0;
   border-bottom-right-radius: 0;
 }
-.btn-group > .btn-group:last-child > .btn:first-child {
+.btn-group > .btn-group:last-child:not(:first-child) > .btn:first-child {
   border-top-left-radius: 0;
   border-bottom-left-radius: 0;
 }
@@ -6167,6 +6167,7 @@ button.close {
   height: 20px;
   margin-top: -10px;
   font-family: serif;
+  line-height: 1;
 }
 .carousel-control .icon-prev:before {
   content: '\2039';

less/button-groups.less

@@ -71,13 +71,13 @@
 .btn-group > .btn-group:not(:first-child):not(:last-child) > .btn {
   border-radius: 0;
 }
-.btn-group > .btn-group:first-child {
+.btn-group > .btn-group:first-child:not(:last-child) {
   > .btn:last-child,
   > .dropdown-toggle {
     .border-right-radius(0);
   }
 }
-.btn-group > .btn-group:last-child > .btn:first-child {
+.btn-group > .btn-group:last-child:not(:first-child) > .btn:first-child {
   .border-left-radius(0);
 }