Fix #11400 and #11431

dist/css/bootstrap.css

@@ -4031,10 +4031,18 @@ textarea.input-group-sm > .input-group-btn > .btn {
   position: relative;
 }
 
+.input-group-btn > .btn:not(:first-of-type):not(:last-of-type) {
+  border-radius: 0;
+}
+
 .input-group-btn > .btn + .btn {
   margin-left: -4px;
 }
 
+.input-group-btn > .btn + .btn:last-of-type {
+  margin-left: -5px;
+}
+
 .input-group-btn > .btn:hover,
 .input-group-btn > .btn:active {
   z-index: 2;

less/input-groups.less

@@ -121,16 +121,26 @@
   &:last-child > .btn {
     margin-left: -1px;
   }
-}
-.input-group-btn > .btn {
+
+  > .btn {
     position: relative;
+
+    &:not(:first-of-type):not(:last-of-type) {
+      border-radius: 0;
+    }
+
     // Jankily prevent input button groups from wrapping
     + .btn {
       margin-left: -4px;
     }
+    + .btn:last-of-type {
+      margin-left: -5px;
+    }
+
     // Bring the "active" button to the front
     &:hover,
     &:active {
       z-index: 2;
     }
+  }
 }