Using latest Delphi (12.1), you can open a new edit window (right click on the tab and select "New edit window". Then in the window which is opened, you see your source code and you can switch to design mode by clicking on "design" tab located bottom right of the edit window. Now, you see the form editor. It is still embedded in the designer window but it is bigger than in the main IDE window yet still not full screen.
You can enter a feature request in Embarcadero Quality Portal. If you do, publish the reference here.