MseIde and Fpgui are separate IDE's and implement their own 'custom drawn' implementation.
This is not correct.
MSEide is an IDE. MSEgui and fpGUI are graphics/widgetset frameworks that are 100% implemented in Object Pascal and supply a full range of custom drawn widgets/controls. fpGUI also includes a customised AggPas backend, which gives advanced 2D graphics capabilities and can completely bypass the graphics painting requirements of X11 or GDI.
fpGUI doesn't force dependency on any specific IDE - that is one of its design goals. You can develop your fpGUI-based applications using Lazarus IDE, MSEide, Geany, Notepad, gEdit, vi etc. Your choice! Yes, fpGUI includes an IDE application, but that is more of an "advanced demo application" - meant to demonstrate more complex usage of fpGUI (because I can't share my commercial work with the public). Maybe someday the demo IDE application will become a fully functioning IDE, but that is not its current intent. fpGUI also includes other handy applications like
DocView (help file viewer) and
UIDesigner (visual forms designer) - but again, you are not forced to use these.