The Component::getScreenPosition() method seems to be returning the wrong value - instead it is returning the component’s position relative to the window, not the screen.
As you can see, getScreenPosition() is returning { 10, 10 } when infact it should return { 670, 330 }.
This is with the latest commit on the develop branch.
The first setSize call in your constructor calls resized() of your MainComponent before it is added to the desktop.
What does a DBG show, after the window is actually on the desktop? Still 10, 10?