I realise this post is a bit old but, just in case others read this, I found a work-around as follows:
- Pop out the display (Right-ALT + Enter)
- While it is still in a window, resize the window so that the black bars no longer show (i.e. make the window narrower - the display will refresh so you can see once it’s all lined up)
- Move the window to the destination display and press Left-ALT + Enter to make it full screen
The image should now fully fit the display.
Also, if you haven’t done so already, check out MSFS Popout Panel Manager to help automate this process.
(Note: This was done with the FlightSimBuilder G1000 (1024 x 768), Windows 11 Pro x64 v23H2 and MSFS 2020 v1.38.2.0)