When an Ogre RenderWindow is resized, an event is fired, and handled by MyGUI:
void OgreRenderManager::windowResized(Ogre::RenderWindow* _window)
So far, so good. But the processing in this listener call "getActualHeight()" and "getActualWidth()" Ogre::Viewport's function which at the moment of the call contains the old size of the render window!!!
It's mandatory to force the update of the viewport width/height using the function "_updateDimensions()".
I'm quite surprised that nobody else had the problem before :/, this issue is really harmful.
The patch file.