I have a very mild jolt of anxiety every time I want to enter a new line in Teams or Slack, wondering if it will send a half completed message I will need to edit after the recipient has seen the half completed message, or it will enter a new line like I want it to.

The behavior also changes if you start editing a numbered or unordered list. Maybe that enters the "formatting tools" mode you mention?

Teams is insane. You want a new entry in a bulleted list? Hit the enter key. If you dare.

I had managed to be on Slack exclusively for at least 10 years. Recent acquisition has me using Teams and it's hilarious to see for the first time what people have been complaining about. I thought surely people are exaggerating. No, no they are not.

It only took a couple weeks for me to figure out that I would have to compose longer messages somewhere else and then paste them into Teams.

The Signal desktop app does both too, I guess, but in a way that actually makes sense. Enter sends a message since IMs tend to be short one-liners. Shift-Enter inserts a line break.

But if you click an arrow on the top of the text box, it expands to more than half of the height of the window, and now Enter does a line break and Shift-Enter sends. Which makes a lot of sense because now you're in "message composer" / "word processor" mode.

Slack is similar Shift enter in normal text. Enter in a code block, shift enter sends in a code block.