The combination of "newlines in pasted text will trigger messages sending in some chat clients" and "the middle mouse button is millimeters away from the spacebar on some laptops" is awful behaviour. I have had mortifying accidents because of it, where multiline text automatically sent to people I was speaking with- because I have human motor skills and hit the middle mouse button by accident.
I admit it *can* be useful, I guess... but it's behaviour I would happily get rid of.