Qt program is suspended when it's running in background window
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You say you've made a program to show a camera image and refresh it regularly... I wonder if when it goes into the background if Windows is preventing the screen refresh from happening because it is not the active window?
To rule this in/out I would add some logging to your program to write out each time it takes an image and refreshes it. Perhaps to do this slow down your refresh rate so you don't get a ton of log entries.
Then run your program and put it in the background. See if you are getting log entries in that state. I suspect you will be and that it is a screen refresh issue. I can't say for sure what is going on.