I've given this a play and I hope you don't mind some constructive criticism I've experienced:

First of, I really like the idea of this project despite normally being a vocal critic against Electron. However I think you bit off too much trying to write your own $SHELL as well as terminal emulator. The result is rather uncomfortable to use because there are shell bugs and incompatibilities compounding the usual expected bugs with any new terminal emulator. For example it took me 5 minutes to just cd into a directory because env vars weren't being recalled, the ~ variable wasn't being recalled, and auto-complete wasn't keyboard optimised (I'd arrow down and hit enter but it would crop the line rather than select the auto-complete entry).

So the next thing I did was to start my preferred shell and I then ran into a whole other series of bugs due to the terminal emulator being tailored for it's own specific shell. In fact the bugs there were so severe that it renders the term virtually unusable on any of the standard shells nor the more esoteric ones I contribute to.

One thing I was pleasantly surprised about though was how well it supported ANSI escape sequences - even ones that aren't part of the formal standard (such as the iTerms true colour sequences). But the bugs surrounding the terms handling of readline (and similiar) really feels like many of the basics are missing despite having some cool advanced features already.

Overall, as much as I'm ideologically against Electron, I can see the advantages of using it in this domain because it allows the ability to include rich content in the command line. It might be nice to get to the point where the command line can offer as much rich content as a web page (something I'm working towards with my own project in fact) - in which case Electron (or similar tech) seems the obvious choice for building a terminal emulator. However upterm's dependance on using your own implementation of Bash (I think it's a bit disingenuous calling your shell "Bash" but that's your prerogative) makes this highly frustrating compared to other rich terminal emulators already out there. And the inability to run your own shell within upterm makes this inflexible for anyone wanting to do something a little more indepth than just cat'ing a JSON file (even running a DB command line tool like mysql or redis-cli would be impractical in upterm in it's current state. And while I didn't try SSHing into remote boxes I can see the same issues happening there too).

I don't know the age of this project but I think it does have real potential. However if I was to ask for anything - and I appreciate this is your baby so I have no entitlement what-so-ever - it would be if you could offer a $SHELL'less version for power users.

edit: Also, I've noticed commands that fail don't appear in the upterm's history. Was this an intentional decision? I appreciate there are arguments for and against only including successful commands but sometimes you'd typo something and the inability to up-array and edit my failed commend added to my frustration. If this was an intentional design decision then is it worth including the last command regardless of success and then excluding it from all subsequent placements in the history?