I have recently removed tabs from my Intellij setup, because with all the features available to navigate between files, who needs them?
The issue is, the normal way of d
The keyboard shortcut your are looking for is SHIFT+F4 (the action assigned to this shortcut is called Open Source in New Window. When you invoke this action the currently opened tab (file) will be opened in new editor window. However the file will remain opened in the main window too, but I believe that this is as close as you can get to the functionality you describe, at least for now.
Open the "Open class" dialog using CTRL+N search for a class you want to open in detached window and hit CTRL+SHIFT+ENTER. That will open the class in a new editor window.
Unfortunately this seems to work only for Java classes and not for other files (CTRL+SHIFT+N dialog). I created a bug report for the last problem, so if you think it might be useful, feel free to vote for it.