Source Editor Maximize
Back in time of Delphi 7 and IDE was not fully dockable. In fact source window of the and Main window of IDE could not be docked on into another. (Actually nothing could be docked into the main ide window).
However, Delphi 7 had a very nice feature. If a source editor were maximized, its top position would always be below the main ide window.
Error creating thumbnail: File missing
That might looked like source editor got docked into the main IDE, or to some extent that IDE is multi-window application and source editor is a child window.
This feature was never a part of Lazarus.