Grow button width to the parent button group (#27717)

Author: ysds

scss/_button-group.scss

@@ -9,7 +9,7 @@
 
   > .btn {
     position: relative;
-    flex: 0 1 auto;
+    flex: 1 1 auto;
 
     // Bring the hover, focused, and "active" buttons to the front to overlay
     // the borders properly