At https://doc.qt.io/qt-5/stylesheet-examples.html#customizing-qtabwidget-and-qtabbar, one can find the following sentence:
For a tab widget that looks like above, we make use of negative margins.
The hyperlink on "negative margins" points to http://www.communitymx.com/content/article.cfm?cid=B0029, which disappeared somewhere in 2015. According to the Wayback machine, the website was describing the CSS box model with a nice diagram for negative margins.
A possible fix would be to point to Qt's own CSS box model reference, https://doc.qt.io/qt-5/stylesheet-customizing.html#the-box-model, and add the following sentence there: Negative values draw the element closer to its neighbors than it would be by default. (Taken from MDN)