missing / cut off scrollbars in different browsers

Steps to Reproduce

https://keikai.io/demo/excel_like

Current Result

FF: (scrollbar left / top border is cut off by content)

Macos: (depending on OS "Show Scrollbars" setting)
scrollbars are completely missing (covered by the cell content)

Expected Result

always show custom scrollbars, or OS/browser native scrollbars.

Debug Information

the current custom scrollbar is using Simplebar, and has a fixed width of 15px (defined in CSS)
the width/height of the data area ".zsscroll.zssscroll-fix" is computed based on the detected native scrollbar width. If the native scrollbar is too narrow the content will overlap the custom Simplebar-scrollbar (partially or completely) or potentially leave a gap in case the native scrollbar would be wider than 15px

Workaround