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It would be nice if Okular and KDE apps in general can support dragging tabs outside of the app's window and make up its own window just like many browsers.

Take Okular for example, I like keeping Okular documents in the same window so I can easily navigate among documents and don't have to go back to my taskbar. However, sometimes you want to documents to be displayed on each side of the screen, so a tab dragging (to make up its own window) can be extremely handy.
It is easy to imagine such scenarios when using other KDE software such as Konsole and Dolphin.