What language are you using? Blitz, AGK etc
I'm not sure i've ever seen a game that uses drag able windows though. Not that i've seen many games
For a game....(not an editor...a game) - most guis probably just needs buttons, text fields, labels, editable text fields, panels and borders, and that's about all really isn't it?
If you are going to have content outside the size of the window, which can be accessed using a scrollbar ie. clipping, I would suggest you do some extensive testing on the scissor commands. In my experience this is the only way to clip the contents of the window other than rendering to a canvas (which would mean handling all the inputs independently).For one i have noticed that the EditBox only clips the text, Not the background or outline.
Also the lists etc have dragndrop support between the widgets.