can't have overflo: visible for "in" which is present during entirity of transition.

docs/assets/css/bootstrap.css

@@ -1941,7 +1941,6 @@ table .span24 {
 }
 .collapse.in {
   height: auto;
-  overflow: visible;
 }
 .close {
   float: right;

less/component-animations.less

@@ -16,6 +16,5 @@
   height: 0;
   &.in {
     height: auto;
-    overflow: visible;
   }
-}
+}

less/test.js

@@ -0,0 +1,11 @@
+var util = require('util'),
+    exec = require('child_process').exec,
+    child;
+
+var total = 0;
+var start = new(Date);
+child = exec('lessc bootstrap.scss', function (error, stdout, stderr) {
+	var end = new(Date);
+	total = end - start;
+	console.log(total)
+});