Hiding the scrollbar in the edit-field of the streams-plugin

Using @saqimtiaz ` awsome Stream-plugin I sometimes have a problem with scrollbars in the editbox.
Sometimes appearing and disappearing scrollbars start to oscillate.
(Though this particular behaviour may be due to the languagetool-plugin of my browser I would like to elimiate the scrollbar anyhow).
Somehow the css-spell seems to be different than for other elements because my standart solution does not work.

* {
  scrollbar-width: none;
}
::-webkit-scrollbar,
  scrollbar-width {
  width: 8px;
  height:8px;
}
html {-ms-overflow-style: none;}