Howdy! Not sure this is the correct section to post this question but I’m very curious about the GUI toolkit used by UnrealEd. Is it just entirely custom? Chromium based? Can’t see Qt anywhere etc.
That would be Slate:
Ah. The “entirely custom” option. 
Cool.
Hi! The toolkit is called Slate. It’s written entirely in portable C++ and uses our Render Hardware Interface (RHI) abstraction to draw. It has very few external dependencies (FreeType2 being the only one I can think of). This makes it possible to port to just about any platform that we want to run on.
If you want to check out what Slate code looks like, you can go to Window->Developer Tools->Widget Reflector in the editor. Then click “Pick Widget” and hover over some piece of UI. Press ESC to stop picking. From there you will see the file and line number that instantiates any of the widgets you want to look at. Have fun exploring!
Awesome! Thanks for the overview and tip on the widget reflector @crackpots!