Based on a dialog with Shadow Wizard, it appears this only occurs when the page is zoomed.
As I don't zoom my pages, I overlooked that Windows actually does, where it has a display setting one can change the size of text, apps and other items.
With this set to 125%, which mine were, it create the same effect page zoom does, hence some posts get a scrollbar.
Update based on a comment
.question-container element has its
overflow set to
auto, and by that is allowed to render a scrollbar if necessary, which it obviously does in some situations.
If one change it, to e.g.
overflow: hidden, it goes away.