It's finally a nice day here, 72 degrees and sunny. I set my phone, in its case, on the porch for less than a half hour, and got the too hot message. Really? What's it gonna do when it's 90 out?
I wouldn't recommend leaving electronics in direct sun unless they are intended to be left outside. Temp is one thing but sunlight changes the equation.
Did nothing with the phone in that half hour except wake the screen up once or twice to check the time. It was in direct sunlight but face down.
If this is going to be normal this phone is absolutely going back. I had my m8 for two years, spent entire summer days outside in 90 degree heat and only saw that message twice in two years.
Seriously, you leave your phone in direct sunlight and expect it to not get hot??? There's no logic there. And the 10 probably has better software protection protocols than the M8 in place.