Preview render updating out of the blue.

Hello,

When using a somewhat longer Cycles viewport preview, I notice the preview render restarts from time to time WITHOUT the user making any changes and even WHILE the preview is still rendering samples… This is bizarre behaviour indeed and interferes when I want to wait for the preview to reach a somewhat higher amount of samples…

Any ideas what might be going on and possible workarounds?

Does this still happen if you unplug your mouse?
I could imagine a defective mouse wheel, which sends miniscule up/down (= zoom in/out) commands in random intervals that trigger the preview update.

It happens on several different computers, and also when the mouse is unplugged.