I have googled this and can’t find any info . is this possible
Hi,
Sorry, this is not possible. The Key Editor is based on the selection in the Project window (not on the track). So every single Key Editor doesn’t know about the content of any other MIDI Part.
You can disable the Show Part Borders, but this will not show the content of other MIDI Part.
The only way is to select multiple MIDI Parts in the Project window. Then you can see the content of all selected MIDI Parts.
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…or to put it positively: Yes, it can. You just need to select the other parts, too. 
The marked area helps a bit when using multiple parts in the Key Editor.
I know, you asked for non-selected parts. But I have my positive day today.
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I am 4 days in on Cubase 12 and I love ! This forum is an amazing help . Thanks to all .
I am not going back to Logic 