I found this popular stackoverflow post (see also jsFiddle), which covers exactly my problem. But after applying this solution to my MWE, see on bootply, it still doesn't work. Why is it not working?
HTML:
<div class="container">
<h1>Hello!</h1>
<div class="row">
<div class="col-sm-3" style="background-color:yellow;">
<div class="outer">
<div class="inner">
<p>hello</p>
<br>
...
</div>
</div>
</div>
<div class="col-sm-9" style="background-color:pink;">
<p>hello once</p>
</div>
</div>
CSS:
.inner {
height:200px;
overflow-y: auto;
}
.outer{
overflow:hidden;
}