I've seen the leak three or four times more yesterday, while in Opera Beta or Dev. Apparently not related to the content of the page, but I might be wrong here. Before that I thought it had to do with Discourse, but it happened also on other pages, even with only "opera:sync-internals" open.
But it is definitely not related to Gmail.
Anyway, I don't care much anymore. In the meantime Opera exposed so many other technical problems and design problems that I moved on to Vivaldi. This thing is felt 50× better than Opera and might be a worthy candidate for my future default browser. (Well, currently it already is my default browser, but still on a trial level.)
Nevertheless many thanks for your replies and support efforts, @leocg