There are two known cases in which the blanking of the remote physical screen doesn't work as expected.
When the NoMachine user connects to the remote computer, the physical display is obscured as expected, but screen flickers and flashes making the desktop content to be shown intermittently.
This problem occurs when one of the following conditions apply:
1) On Mac - when the 'Automatically adjust brightness' option is enabled.
To avoid the screen blanking flickering, it's necessary to disable this option. You can do that through the Apple menu > System Preferences, click Displays.
2) On all platforms - when a program (e.g. f.lux) for adjusting automatically the screen brightness according to the ambient light is in use.
To avoid the screen blanking flickering, it's necessary to disable such program.
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