To reproduce, set up Mac Spaces with multiple desktops. Then:

1) Build the examples/widgets/painting/basicdrawing project.
2) Run the application.
3) Right click on the application icon in the Dock. You should see "Options".
4) Under "Options", select Assign to "Desktop on Display 1"
5) Move the application to the other Monitor.
6) Click on the "Pen Join" combo box.

-> the popup opens on wrong monitor