Fixes #11659, #12698, and #12349: Always apply position and z-index form controls in input groups for proper borders

less/input-groups.less

@@ -17,6 +17,11 @@
   }
 
   .form-control {
+    // Ensure that the input is always above the *appended* addon button for
+    // proper border colors.
+    position: relative;
+    z-index: 2;
+
     // IE9 fubars the placeholder attribute in text inputs and the arrows on
     // select elements in input groups. To fix it, we float the input. Details:
     // https://github.com/twbs/bootstrap/issues/11561#issuecomment-28936855