By default, scroll bars do not appear on Mac except when the user is scrolling and when there is hidden content. You can double check this by going to System Preferences -> General -> Show scroll bars: Automatically based on mouse or trackpad.
When you do scroll, the width of viewport and the available width of the inner content does not change from what it was. If the width was 300 pixels, it still is 300 pixels.
If you change the "Show scroll bars" setting to "Always" then the scrollbar takes up a decided amount of width - 16 pixels to be precise.[^1] Let's say your browser height is 300 pixels and your broswer width is also 300 pixels. With this setting, if you toggle the height of the body from 300 to 600, causing scrollableness, then a scrollbar will appear only have you have done the toggle. The width of your body will have gone from 300 to 284 pixels, because the scrollbar takes up space in a way that it did not in the other setting. Interestingly, if you're talking about