Tree headers are scrollable in default state

Open the example provided in the readme.

Set "treeStyle" to the following:

```
const treeStyles = {
color: 'white',
fill: 'white',
width: '50%'
}
```

You'll see a scrollbar on collapsed nodes:

Collapsed elements are scrollable. Scrollbar disappears after expanding element.

Modified example, which reproduces the problem: https://codesandbox.io/s/y0rozoy2yx