I know, I know, but haven't you wanted to jot down some pseudocode while out and about, formatted neatly, so you could pop it over to your main machine to turn into working code?
Then probably Pydroid 3, as it supports TK, Kivy etc. If you actually want to compile and run various languages you could use Termux, as it basically exposes the fully fledged Linux machine better known as your Android.
Whatever notetaking app you use? If it's just jotting down some things I came up with on the go it won't be anything I need a whole ass IDE for, and I don't think I do anything like that often enough to need even a lightweight plaintext editor for that. I guess you could try use vim with termux, sounds like a pain in the arse with a phone keyboard though...
When I'm forced to edit text on my phone (eg. to fix a broken server while on the go), I ssh in and fire up ed.
This is what takes the less screen space on my already to small screen, and because it's line oriented the screen doesn't bounce/resize/screw up when the keyboard appears/disappear.
I wanted to mention a new open source project Xed-Editor.
Additionally, I think the (virtual) keyboard you use is just as important as the editor (assuming you aren't using a physical keyboard). I would recommend Unexpected Keyboard I have had a great experience with it.
I'm going to try Unexpected. The swipe for symbol reminds me of my favourite on-screen keyboard, Fitaly. Unfortunately, nobody ever made it available for modern smartphones.
Edit:
The main thing I see missing is the option to swipe for uppercase. That may be possible in settings, but I didn't see it in a quick glance.
I did do some during downtime on the deskop at work. so it wasn't quite as extreme as it sounds. the worst part was the limited width of the screen. A bluetooth keyboard would have helped a lot, that's for sure.