for some time (since webix 6.x) there ist stage behavior in data tables with horizontal scrolling.
https://snippet.webix.com/2sebqcjx
when you scroll and then edit the combo and select a value sometimes header and data scrolling is different.
(Safari on Mac)
same in vertical scrolling
https://snippet.webix.com/8g425ic9
when you edit the combo
(only appears if screen is smaler than content)