Select Widget in TitlePane is not properly shown

**Bug**

An expended select widget inside a title-pane is shown incorrectly, parts of the select box are not shown.

**Browser**
Chrome 68, Windows 10.

**Code**
```
w(TitlePane, {
title: 'Dojo Select in Title-pane',
key: 'select-title-pane'
}, [ w(Select, {options: [ 'foo', 'bar', 'baz', 'qux' ]}) ]),
```

**Expected behavior:**
Select box overlays title pane.

**Actual behavior:**

Select box is partually hidden.
It looks like the problem is in the
`overflow: hidden` defined in the titlepane.

**Closed:**

**Open:**