if you look at IGraphicsCarbon.cpp - IGraphicsCarbon::Resize(int w, int h) you will find the platform specific code for GUI resizing, I'm not sure how this relates to what you do with frameworks other than IPlug.
https://github.com/olilarkin/wdl-ol/...hicsCarbon.cpp
Also the VSTGUI mailing list has some clues about this stuff I think.