I could not help but notice - and I figured that I should put some credit where credit is due - that tab dropping works pretty well Cross Browser.
I can drag my tab over from Vivaldi to Chrome and it works. Well, it will lose session data etc, but that is clearly to be expected. And I can drag it over to Firefox, and it works. I can drag it over to Opera 42, and it works. Sweet. If I drag it over to Opera 12, it will almost work - it shows a line between tabs where it'd be put, but it doesn't actually open a tab there.
It doesn't work the other way. You can't drag a Chrome tab to another browser, not even the Opera clone. You can't drag a Firefox tab to another browser. And you can't drag a Opera 42 tab to another browser.
Here's the funny little oddbit. Revisiting the old turf, Opera 12 seems to have this feature. I can drag a tab from Opera 12 to Opera 42, or Chrome or Firefox, or Vivaldi, and it works. In that little, age-old browser-that-could.
And now it works in Vivaldi. You can really see where the heart of Opera went.