niceditor problem

Hello
I caught in problem while making niceditor with webix.
https://snippet.webix.com/r4ib9l37 is my code.
The code I have done seems worked great.
However when I click the select box such as ‘font Size’ and scroll the page down,
it seems like select box follows the sroll and moves to the wrong place.
Is there any way to fix location of my editor’s select box?

Thanks for your support!