Messed-up left margin has appeared

I have a newer 2019 MBP 16" using Mac OS Catalina 10.15.7 (Purchased about 2 months ago)

If you use the touchpad and do the 'two finger expansion', it will expand everything according to how far (great) you expand everything on the screen.
Of course, you can also just as easily contract or downsize everything by doing the opposite with your fingers. I don't know if THIS is what you are doing and/or experiencing, but just my 'two cents worth'.
BTW: I previously had a 2012.5 MBP and it was more intuitive and easier for ME, kind of like an older vehicle with more 'Hard Buttons for controls" ...

I have a newer 2019 MBP 16" using Mac OS Catalina 10.15.7 (Purchased about 2 months ago)

If you use the touchpad and do the 'two finger expansion', it will expand everything according to how far (great) you expand everything on the screen.
Of course, you can also just as easily contract or downsize everything by doing the opposite with your fingers. I don't know if THIS is what you are doing and/or experiencing, but just my 'two cents worth'.

That two-finger expansion is different because everything expands beyond ("underneath"?) the edge of the browser window. Everything stays the same (horizontal menu, "Home" button", no black column) but is just bigger. You can then slide it around to see stuff that is actually still there.

BTW: I previously had a 2012.5 MBP and it was more intuitive and easier for ME,  kind of like an older vehicle with more 'Hard Buttons for controls" ...

Yeah, I just got mine, a 13" version, last May or thereabouts. I think I like the Touch Bar strip most of the time, but I'd rather have a real Escape key.

One mis-feature of the MBP that annoys me is that the Speakers/Headphone switch is controlled not directly by the jack contacts, but by the USB bus. Some (non-Apple) apps are not smart enough to switch automatically unless you select it in the app or the menubar. eg. With VCV Rack, if you pull the headphone plug out without selecting internal speakers first, and then try to select speakers, it will hang with the rainbow beachball cursor. The Audacity audio editor does not work with my Griffin iMic in the Catalina OS, but I can boot it up in Mojave (on a separate APFS volume, for running legacy 32-bit apps) and it works fine. At least it works when I use it for Zoom (in Catalina). I'm annoyed at a lot of the app changes too. At this rate this MBP may be my last Apple hardware purchase (first one was 1986, a MacPlus).

But I digress.