This is what I did: this works on chrome on windows 10.
1) Log into the browser version of teams: teams.microsoft.com
2) open up the particular chat window that you want to copy by selecting ‘chat’ and the person.
3) start scrolling up in the chat window until everything you want to copy is loaded off the server.
4) Install https://gofullpage.com/
5) you need to get rid of all scrollbars except for the actual ‘chat’ you want to copy.
my conctacts list had a scrollbar, so I had to get rid of it by clicking on the bold face ‘CHAT’ and selecting ‘contacts’
6) screenshot the page with gofullpage.com by doing alt-shift-p.
Note the lower the screen res is, the harder time any OCR tool will have to decode the text.
However if your chat is too long and/or your resolution is too high, then I think gofullpage will blow up.
7) save the capture to pdf (and go ahead and save the images to, if you know of a clever tool)
8) Convert the pdf to text. One way is to load it into microsoft word which does an autoconvert.
word makes lots of mistakes, but you can always cross reference back to the screenshots (using the dates) if you need to