title: Enabling `overflow: scroll` for a parent object makes a scrollable child object unreachable
body: <p>In Handsontable, there is a DropDownMenu object. I've managed to decrease its height and add the CSS-property <code>overflow: scroll</code>. Also, there is a child object CheckboxList (CheckBoxRenderer) inside it, which is also scrollable (using the same property <code>overflow: scroll</code>). </p>
  
  <p>The problem is, when I make DropDownMenu (parent) scrollable, I cannot reach the CheckboxList (child) - I can't neither click a checkbox, nor scroll the child object.</p>
  
  <p>Is there any way to add some property like <code>: focus</code> or <code>: active</code> to reach the child object?</p>
  
  <p>Alright, here is the catch: </p>
  
  <p>This is a child object</p>
  
  <pre><code>.htDropdownMenu .ht_master .wtHolder {
    overflow: scroll;
  }
  </code></pre>
  
  <p>This is a parent:</p>
  
  <pre><code>.htDropdownMenu .ht_master .wtHider {
    overflow: hidden;
    width : 130px !important;
  }
  </code></pre>
  
  <p>Now, everything is OK - child object is scrollable and runs through all existing rows in the list properly, as well as clickable, too.</p>
  
  <p>Let's change them a bit. I want do make a parent object smaller, so I add height into the class style. Because of this, some objects get hidden due to an overflow, so I add a scroll and here's the result:</p>
  
  <pre><code>.htDropdownMenu .ht_master .wtHider {
    overflow: scroll;
    width : 130px !important;
    height: 130px !important;
  } 
  </code></pre>
  
  <p>This results in parent object become scrollable, but the child is not scrolling (despite it has a <code>overflow: scroll</code> property)</p>