[jimbokun]

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?

[dfxm12]

If you're worried about this, consider a copy and paste from notepad.

[abraxas]

Your advice beautifully reinforces OP's case. We now have to use auxiliary apps with more predictable behaviour because designers made such a mess of things.

[oneeyedpigeon]

We've still yet to reach the "any textbox is just an embedded version of your EDITOR" nirvana.

[jimbokun]

Note the "very mild" jolt, not quite enough to rise to the level of switching to Notepad (or Emacs in my case).

[fireflash38]

Shift enter should always put in a new line. Good luck exiting a block though with it.