It seems the days of the 60Hz phone screens will be over sometime in the immediate future, now that 90Hz phones are starting to gain more traction. The Pixel 4 is the latest phone to offer the faster refresh rate, and smooth browsing that comes with it, but there's a problem. The Pixel 4 doesn't display at 90Hz if you're using it at low brightness.
This 'quirk' has been discovered by intrepid users over on Reddit and XDA Developers who found that if the Pixel 4's screen brightness falls below 75% you get a sub-90Hz refresh rate. The only exception, apparently, is if you have very high ambient brightness - though if you're in a very bright environment you probably shouldn't have your phone screen's quite so dim. Not that 75% of maximum is particularly dim, of course.
Changing refresh rate based on usage is pretty normal, since it helps preserve the battery. As TechRadar notes, basing it on the brightness of the screen hasn't happened before. Thankfully there is a workaround, though something tells me it might make the Pixel 4's dismal battery life even worse.
To keep your 90Hz up you need to activate the developer options: go to Settings > About Phone then tap the Build Number until the phone tells you you're now a developer. Developer options then appear in the 'System' menu, and in there is the option to force the 90Hz refresh rate. It's a couple of options above the networking sub-menu if you're struggling to find it. [Reddit, XDA via TechRadar]