Apparently this hanging of the program in Sources is a peculiar problem dependent on the size of the main window as I’ve discovered after reading this report from Rick Matheson: RootsMagic Users | Facebook
After reading it, I revisited my test and find that maximising the window clears the freeze-up while reducing it triggers the fault. Reducing it below about 2/3 screen height (1920x1080 monitor, 100% scale) is the onset; width does not seem to matter.
The same thing happens on my smaller laptop monitor (the two are stacked vertically), also 1920x1080 but 125% scale because it is smaller. Dragging the frozen window from the laptop monitor up to the 2nd monitor can also trigger the same error and results in a partially filled window:
Repeatedly dragging it up and down between monitors and sending error reports eventually restores the window to normal functionality.