For some reason, maybe because I accidentally pressed something on my keyboard, Visual Studio 2019 started comparing code in inline mode. By inline mode, I mean I only got one code comparison window, and the changes were stacked on top of each other. This made it hard for me since I was so used to the side by side code comparison mode, where I had two windows and I can clearly see what's different between files.
It took awhile for me to figure out how to set it back. I spent some time looking through the options in Visual Studio but couldn't figure it out. I even had a hard time searching the internet for answers, because I wasn't sure what to search for. The search key that finally worked for me was "vs 2019 diff inline mode". That led me to the Stack Overflow post I linked below.
Sidenote: I find it hilarious that the question in Stack Overflow was for Visual Studio 2012. Here I am with Visual Studio 2019 and the answer still works. Amazing. And what a gem Stack Overflow is for software developers.
Anyway, there are two ways to set it back to side-by-side mode. You can use the shortcut button for it in the menu bar. Or, you can use the shortcut
CTRL + \,
CTRL + 2.