Window keyup not working

Hi
What I want to do is allow the user to hold down a key to briefly show a 
value in a Label.
In a new project, this works:

KeyDown(Key as String) as Boolean
     If Key = "c" then
         Label1.text = "the info"
     End if

KeyUp(Key as String)
     If Key = "c" then
         Label1.text = ""
     End if

In the app I'm working on, KeyDown works fine, but KeyUp never fires 
(with or without "return true")
None of the controls on the window have a KeyDown nor a KeyUp event handler.
All of the controls on the window are either canvases or labels, many 
include MouseDown and MouseUp event handlers

Does anyone have an idea what sort of thing might be interferring? I 
don't know where else to look.

Xojo2017r2.1
Linux Mint 18 and also Debian 7

Thanks for your time,
Don


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