scroll of ui.sidebar, Properties Webix Docs

enables/disables the scroll bar

boolean|string scroll;

"y" (or true)

enables vertical scroll

"x"

enables horizontal scroll

"xy"

enables both scrolls

"auto"

enables scroll only if the content of the list is bigger than the container

false

disables both scrolls

webix.ui({
  view:"sidebar",
  data:menu_data,
  scroll:"y"
});

false