The thing is that the looks of scrollbars vary so much between platforms - on mobile you can barely see them only when you do the action of scrolling! So it would be rather weird that, if you are seeing a website in mobile and you just put a scrollbar because reasons, you're quitting some horizontal/vertical space to the effective area of the website.
And note that the act of scrolling is much more related to the OS host rather than something the website itself should provide (now, if there was an standard way across every device to scroll to the very top or the very bottom of a view (webpage, list, etcetera...). I am a designer myself but I agree than we should not mess with them. And the plus that it's a bit less of work for us (and devs, when they have to implement those things).
It’s an Electron app for me so it’s only going to be on Chromium on desktop
But if scroll bars can be themed with CSS now, that should still give the browser control over things like whether they take up space or not and the actual scrolling behaviour, right? It would just allow the developer to give some styling hints so it doesn’t look completely disconnected from the rest of the page
But it will look disconnected from every other app on the user's system.
Whatever style the user currently has installed on their system, if the scrollbars on your app look the same as all the other scrollbars they ever use, they'll recognise them instantly and know how to use them instantly, with zero additional cognitive load.
You users might be using enough of their brain to figure out your app (maybe because they're also being partially distracted by their boss/child/phone/pet/music) already, don't make them spend time thinking about your gorram scrollbar as well.
And note that the act of scrolling is much more related to the OS host rather than something the website itself should provide (now, if there was an standard way across every device to scroll to the very top or the very bottom of a view (webpage, list, etcetera...). I am a designer myself but I agree than we should not mess with them. And the plus that it's a bit less of work for us (and devs, when they have to implement those things).