getScrollbarWidth fails in Firefox under certain conditions

[real-artswan]

I have an app where body height is 100vh and children fill the whole height, in this case "outer" element created by getScrollbarWidth is out of browser view port and it seems Firefox doesn't draw any scrollbars in it so inner and outer widths are the same.
I'm able to fix it by adding position absolute to the outer style:
outer.style.position = 'absolute';
Could somebody publish the fix to the npm, please? Or should I make a pull request?