blue marked region appears in other window of C-x 2

[Dan Jacobson]

Gentlemen, I have emacs set up so regions I mark are highlighted in
blue.  When I split the file into 2 windows and make a mark and
proceed to move downwards, creating a region, why does some blue
activity occur in the other window?
$ emacs
C-x i / e t c / t e r m c a p <return> C-x 2 C-v 
C-v C-v C-v C-v C-v C-v C-v C-v C-2 C-n C-n C-n C-n 
C-n C-n C-n

[The C-2 sets a mark for me.]

By the way, I left that other emacs in that state while writing this
message and now suddenly minutes later it begins munching CPU, so I
got rid of it.
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