By “suddenly”, I mean starting a couple of weeks ago, so it could have been introduced by an update. But I wasn’t seeing anybody else posting about this problem, so I presumed that I had accidentally changed some setting. However, I couldn’t find any pertinent setting.
And then, on another machine, with a brand new (i.e. first ever) installation of the Brave browser (1.50.114) I’m getting the same result: no scroll bar.
But…
On a third machine, with the same version as the first one (1.49.132), the scroll bar is present! 
So… it must be a setting, right? Where is it?
BTW: These kinds of desktop issues are usually not OS-specific, so I wasn’t going to tag it as “Linux”, but a tag is mandatory. 