Over the last week or so, the screen on my MacBook Pro has been randomly shrinking to about 3/4 of it's normal size. This lasts a couple of seconds then it returns to normal but will do it several times in succession then is OK for a while. Tends to be when I'm trying to type something online, eg Facebook, Whatsapp etc (on Chrome). When it shrinks it has a blue border and I have to click on the screen each time to return it to normal. I've tried shutting down, restarting etc but doesn't seem to make much difference. Anybody any ideas please? It's doing my head in! Edit - video added.
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