Adding to this, what could be even better would be adding options to manage this when you right-click on a tab title. Notepad++ has this feature and it works extremely well. For example, a tab menu with options:

Move to split screen Move to new window Copy to split screen Copy to new window

An even more advanced option would be move to or copy to an existing instance. Listing all open Opera instances with the name of the current tab would work well.