Hi,
if I click on a mailto-link in Vivaldi nothing happens. I could locate the problem in xdg-email which is used by Vivaldi to start the E-Mail program.
I checked in the terminal and this is the output of xdg-email:
I hope I’m not too late to the party but I just discovered why my Chromium and Vivladi were not working. There is an entry in ~/.config/emaildefaults called EmailClient[$e] and mine was set to thunderbird.desktop. When I changed it to just thunderbird my mailto links started working. Ie; xdg-email mailto:my@email.address opened up in Thunderbird.