In Windows 10, on a system with a 4K display, the top ~20 pixels of every window is cut off, including the main window’s menu bar.
Can you screenshot the output of a Monitors DAT with all the columns?
For some reason it now seems to be doing the opposite, adding a block of pixels, but only when on the screen with lower resolution.
This one is on the 1920x1080 monitor:
This one is on the 3840x2160 monitor:
I can reproduce your case where the 1920x1080 monitor had some black pixels at the top, so I can see that at least here.
I wonder how it would behave if you changed your Windows display layout to all be positive. Your current monitor layout (managed in the Display Settings Window) has your 1920x1080 monitor to the left of origin so the positions are all negative. Can you reconfigure so everything is in the positive and lined up correctly? Windows display bounds sometimes behave strangely and this negative location might produce issues, its is worth trying as a test.
Also I would try to set both monitors to run at the same refresh rate, you currently have different refresh rates which is not desirable.
I tried rearranging the display layout so that the primary display is the 3840x2160 and it’s all the way on the left. It didn’t solve the issue or change the behavior.
This is with TD on the 3840x2160 (cut off):
This is with TD on the 1920x1080 (no bug):
I also tried making the 1920x1080 the primary monitor, and swapping the layout with that one as the primary, all of which showed the same behavior (cut off on the 3840x2160 and fine on the 1920x1080).
With the refresh rate, both windows settings and NVIDIA control panel show both displays as 60Hz, so it’s odd that TD is showing one at 59.997.