I’ve googled for a similar problem but I didn’t find anything.
I have a dual screen configuration (primary is WQHD 2560x1440, secondary is UHD 3840x2160).
When I activate second monitor full screen in Lightroom, the second monitor window does not fills the screen completely:
I’ve tried several combinations, but nothing really works. The only solution seems to deactivate full screen and manually expand the secondary window to screen edges and “simulate” full screen. I never had this problem with my previous configuration (WQHD + WUXGA 1920x1200). Is it related to the quite large secondary screen resolution?
I finally figured out a solution to my problem. I've just ticked the "Disable display scaling on high DPI settings" on the lightroom shortcut. The secondary fullscreen now fills the monitor, the displayed picture is crisp but text is quite small. After one week playing with two screens of different DPIs (normal/high), I can say that Windows is not fully compatible with this kind of setup . I really considering now replacing my WQHD monitor with another UHD monitor to have a full high DPI configuration, which seems to be the only way to benefit a nice scaling and compatibility.