I set a breakpoint in the 'EDIT' section of the StartParamEdit method. The value of pt is exactly the same on the first and second attempt to edit the field, but the box pops up in different places.
I have tested on Windows 10 and XP and the box position is always correct. But on Windows 7, it is always incorrect, on two separate physical machines and one virtual machine, all running Windows 7 Pro 64-bit. This is using the same EXE for all tests.