Bill,
That is a resizable screen capture box (note the handles on each corner, with a 
menu bar below the highlighted area.
There is a small “x” at the end of this menu bar pressing this will cancel the 
action.
The menu bar has rollover tool tips. 
This is a powerful and relatively unknown feature of macOS.

Regards,

robin

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> Sent:
> Thu, 6 Feb 2020 18:02:49 +1100
> Subject:
> Screen problem
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> 
> Hello
> For some unknown reason when I do a screen capture Shift command 5 I am left 
> with a bright area when the capture was done and the rest of the screen 
> darkened, the only way to clear is to do a reset.
> Mojave 10.14.6
> Imac 21" late 2013
> regards Bill

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