I have to open and close the external window before my QML gui is opened.
So I do this in my main before the QML engine loads the QML file.
Then I pass the necessary information from the external window via pointers to my GUI.
I did some exploratory debugging and found that (on Windows 10) QWindowsStyle::drawPrimitive() is not being called to render checkboxes, but rather they are being drawn with DrawThemeBackgroundEx() from uxstyle.dll (which I would have expected to support High DPI correctly.)
After looking at the screen shots attached to the ticket referenced above I discovered that I have a different sort of drawing problem; checkboxes are being render correctly, but at the same size (13px) regardless of device scaling.
On Windows 7 checkboxes appear to render correctly with device scaling enabled.