Why does the display of the browser window size change when the body is removed on any sites and in any browsers?

Question

Browsers

Hello!
I noticed a strange thing while doing this. I turn on the developer tools, and depending on whether the page has a body, the size of the window is shown differently (if there is a body, it shows that the window is 18px smaller. As soon as I delete the body, the size does not actually change, but the correct value appears in the size indicator, t .e. 18px more). This happens on absolutely all sites (habr.com, etc.) and in all browsers, how is this to be understood?

Answer 1

Scrollbar width?