Strange bug …
When the window is maximized, the position given in the mouseDown method is wrong. For instance, the position given is (3 ; 0) where it should be something like (8 ; 7) (my measurements are not precise).
My code works perfectly with JUCE 7. With JUCE 8, it works also when the window is not maximized.
With JUCE8 on MacOS, my code works perfectly when the window is full screen.
Thanks for reporting, it looks like this issue was introduced along with the big window handling rewrite that happened just after JUCE 8. It should be fixed here: