General Discussion

DU does nothing of the sort.

Does it happen in other text areas (boxes) you might encounter?

(I use Hacker News often and it uses the same kind of text area.)

My best guess: You have something in the "copy" buffer. That's fine, There's almost always something there to paste.

Why would your browser offer to paste?

Do you unknowingly have some key pressed, or trackpad touched?

Do you have the control key pressed? The right mouse button pressed?

I have *never* seen "paste" offered, though I very often wish that the phone had a paste button or key.

Infinitely improbable answer: some mouse or keyboard is paired with your computer that you don't even know of.

I've seen (ALMOST) every kind of problem pop up.