It seems this one happens in both github and microsoft sites. You can add a phone number to the account so that you can use it to reset password or do whatever. In case of ms and github you select the country code from a combobox (it seems it's a combobox) and then type the phone number. The problem here is that the list of country codes appears completely empty to me even though it contains entries. Happens on chromium version 80 and 81, but not on firefox. Not sure how to reproduce at this moment with something else.
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