The root cause is that tag is not submitted in Electron as soon as tag editor looses focus (clicked out).
See video (make sure to have player controls hidden - they cover the area where fun takes place). It's possible to enter a text as a tag, not submit it, switch between notes, and have this text shown instead of 'Add a tag...'
Huh, interesting find. I'm not sure if this is a bug or not. If you've entered something in that text input, it isn't cleared on switching notes. I guess we should probably clear it.
I definitely don't think that we should submit any unfinished tag as a tag when you switch notes, that seems a bit unexpected to me.
I agree it's hard to consider this a bug, I'd say this just looks a bit weird. In native and iOS the tag is submitted in this case though...
I was taking a look at this and agree it feels weird the way it is. Working as native and iOS do makes sense to me. If I have typed a tag on a note, but not submitted, and then click out of the tag field it seems like that tag should be submitted. To me clicking away means I'm done inputting now.
When in doubt, making it consistent with the other platforms is usually a good bet.
[testing] fix/feature verified on Dell Latitude 7400 | Win 10 | Simplenote 2.7.0-beta1