Long story short:
I have a mid 2011 imac27, High Sierra. Latest NoMachine version.
I installed various Linux distros on it but often the NoMachine display is utterly garbled and pixelated.
I reinstalled High Sierra
Both my backup (which did work) and a new install show File Manager display !!!
Clicking any entry shows the usual (icon) display. Choosing any host shows File Manager. Clicking any entry shows the usual target, un-garbled.
What might be done about the linux display? If it was l library issue, as suggested by Google, would the display be always garbled. Often ‘quit’ and ‘restart’ fixes the garbled display.
James