I updated to Safari 13.0 (13608.1.49) today, on my iMac 5K、High Sierra 10.13.6 (17G8030), and saw a brand-new problem. (I’ve never seen this in any earlier version of Safari.)
Open Gmail https://mail.google.com/mail/u/0/?shva=1#inbox in Safari 13.0.
Open a just-arrived e-mail message titled “TidBITS Japanese Issue #1482.”
In earlier versions of Safari, I normally (that is, every week) would:
click the Three-Dots (More…) button on the Gmail page window in Safari,
select ‘Print’ from the menu (which opens a new tab, and then opens the Print dialog),
click the ‘Cancel’ button on the Print dialog,
and then be happy with the Safari tab containing the e-mail message
(that is, a full TidBITS issue) only --> Select All and Copy and everything.
However, Safari 13.0 doesn’t let Gmail open a new tab, but instead shows an error message, saying you must enable Pop-up Windows for Gmail, to open a new tab
for the ‘Print’ command.
I opened Safari’s Preferences, went to the "Web sites’ tab, selected ‘Pop-up window’ from the sidebar, and ‘allow’-ed pop-up windows for ‘mail.google.com’.
After that, the problem completely disappeared.
Conclusion: As of Safari 13.0, you must authorize the pop-up window for ‘mail.google.com’, in order to use the ‘Print’ command of Gmail.