Hi, sorry for delaying in testing it...
I can confirm the bug on FreeCAD_0.19.17072_x64_LP_12.1.2_PY3QT5-WinVS2015.7z
OS: Windows 10 (10.0)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.17072 (Git)
Build type: Release
Python version: 3.6.8
Qt version: 5.12.1
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/United States (en_US)
I don't know if this may help...
I noticed that this issue is present also if I UnDock the Document window, instead of calling a F11 fullscreen option.
This is related to Qt5 as you pointed out, because I don't have issue on the same release, but Qt4 based.