Tree headers are scrollable in default state

[KAbakumov]

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

[KAbakumov]

After some investigation I made the following conclusion. Style combination:

left: 40
width: 100% 

Is a workaround to hide the scroll bar outside of the page. However, even in the example, provided in the readme, you can scroll down tree headers and get the following result:

[drcmda]

@KAbakumov it's probably missing an overflow: hidden somewhere. Would you be up for making a PR?