Some details:
This is Linux Zoom 7.1.5. Previously I had 6.6 installed, which didn't do this.
It appears to only care about the CLIPBOARD selection – the one with Windows-ish UX, which you typically copy to explicitly with a keystroke such as ^C, and paste from with ^V (or similar). It doesn't look at PRIMARY, the selection used for trad X11 "just select in this window then middle-click in the other" copy-paste.
It looks as if it's detecting changes of clipboard via the XFIXES extension, and whenever the clipboard is claimed by a new owner, it immediately sends that client a paste request.
I noticed it because I make heavy use of a "one-shot paste" tool which fulfills a single paste request and then terminates. Handy for filling in lots of fields of a web form – queue up pastes of several different things, then go to each form field in turn and just hit paste, bam bam bam. But today, after an apt update, it doesn't work: my one-shot paste tool terminates as soon as it starts, because (I found out after some debugging) Zoom always requests a paste immediately.
(Regular readers might recall that around this time _last_ year I caught Slack doing something similar, but only when focus entered the Slack window, and also, they had a configuration option to turn it off. A one-shot paste tool seems to be a good way to notice weird things going on in the X server!)
