Linux Lite
Maybe I should be able to let go of this distro, but there was one more Displays property that I could try changing, and it might help a lot. In VirtualBox Manager, I'd used the default VMSVGA video support. For one absolutely last shot at this, I tried changing that to VBoxSVGA instead. And suddenly, all of the window sizing problems disappeared. I can resize any application window with the mouse now, including the package manager.
Speaking of the package manager, I've noticed something about my attempt to install the vivaldi.deb file that I downloaded. In this image of the downloaded files installer, this file looks disabled to me, and I haven't been able to change that. Clicking or double-clicking the file doesn't do a thing. And clicking the [Open] button is obviously meant for the package manager, as it displays a list of all the installed packages. At the same time, I can't imagine that Synaptic would exclude a file with a .DEB extension without even attempting to process it.
So I'm going to delete that file, browse to vivaldi.com, and download it again. There's a non-zero probability that Synaptic will open it on the way in this time, and install it. Slim chance, as I said, but that's all I need to try it again. Stand by ...
... Well guess what I just found as I started to delete that .DEB file. Especially take a look at that first line. Apparently Synaptic Package Manager won't do what it's supposed to do, but GDebi seems to be an alternative that might work.
No foolin'! Now I'm getting excited again. I can't wait to click [Install Package]
As you might expect, this unexpected bit of success has changed my perspective completely. Having apparently fixed the window manager issues, and figured out a way to install my browser, I'm once again enthused about giving Linux Lite a full daily-driver test for at least a day.
I have more to configure, though, especially with the Vivaldi that I've just now managed to install. This day is winding down for me, so it's going to be a project for tomorrow morning. I came this >< close to bailing out of this whole project a short while ago, until I quite accidentally figured out that a second package manager existed that might make a difference.