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Interesting... so the black spot goes away when you remove it from the sunlight for a while?

I'd say, if it bothers you then I'd exchange it for another one.

Put off your screen protector. This may help.

This is the first report I've seen of a dark spot on the screen (there have been some reports of darkly-lit spots with the backlkight on, but I think that's different, because your dark spot appears without the backlight illuminated).

It would probably bother me enough to send it back for a replacement.

Keep in mind that this solar display thing is brand new technology, as far as I know this is the first watch ever to use it, and it's definitely the first Garmin to use it. So there are bound to be some issues with some of them. I think this watch is a good candidate for a replacement.

Only sorta related but I've had a Citizen watched that's solar powered only for several few years now.  It's not a smart watch so it doesn't use much power, just average normal while collar daily exposure to inside or sunlight was enough to keep it running without any issues.   Only point to that is it's not necessarily new technology although Garmin may be using newer versions of the technology (I'd be surprised if they weren't). 

Is it a solar panel behind an LCD screen?

If the watch get to hot, the solar charging is stopped automatically and a red thermometer sign is displayed on the display. Have done several solar chargings with laying the watch in direct sunlight for testing the solar functionality, but never saw a black spot on the display.

www8.garmin.com/.../GUID-E72F3619-D372-44BE-BEBC-348065CEAC07.html