fixes #8937: align the dropdown menus in input groups

dist/css/bootstrap.css

@@ -1894,6 +1894,7 @@ input[type="button"].btn-block {
 }
 
 .input-group {
+  position: relative;
   display: table;
   border-collapse: separate;
 }

less/input-groups.less

@@ -5,6 +5,7 @@
 // Base styles
 // -------------------------
 .input-group {
+  position: relative; // For dropdowns
   display: table;
   border-collapse: separate; // prevent input groups from inheriting border styles from table cells when placed within a table