I normally install updates for Notepad++ as soon as they pop up as available, so I think I was updating directly from 8.9.0 to 8.9.1, but I’m not 100% sure. And no uninstall or deletion of settings was performed beforehand; I just did a direct in-program update.
Good news: my project is up and running again. So, crisis averted. And in the process of making that happen, I think I figured out what went wrong. Some of this was definitely user error on my part, as you’ll see below.
It’s my theory, that when the program updated, for some reason, it deleted some of my custom settings (which I perhaps didn’t realize were custom, as they’d been in place so long). This might include the Python syntax settings for colors & style, as well as unindent behavior of the backspace button (perhaps indentation settings in general). When I tried to re-enable the unindent behavior, I only enabled it under Default, but not under Python, so that may be the source of some of the inconsistent results I was seeing. (Results seem to be consistent now with it enabled under Python.) As I was trying to figure things out, and get my settings back to where they were, I was also writing code in between these attempts, so I think that resulted in some of my code having mismatched indentation styles applied: I think this caused the IndentationErrors & TabErrors I was experiencing. (Which, thankfully, are now resolved.) And I think this was also causing havoc with the folding behavior of my code, as now that also seems to be cleared up. (Cross fingers.) Thank you for the tip on the View > Show Symbol > Show Space and Tab option. I wasn’t aware that was available. It’s very helpful.