Adjust the brightness of SDR content on your HDR display: Starting with Build 17040, Windows now lets you adjust how bright SDR content appears when running in HDR mode on the desktop. On an HDR capable system, you will see a slider in the “HDR and advanced color settings” page under Settings > System > Display. This is one of the improvements to HDR image quality that we have planned based on feedback from Insiders.
This one was a long time in the making as MS should've added this control back in 1703 already but it should help in situations when turning on HDR in Windows results in a completely unusable desktop.
For those unaware of the issue, Windows 10 1703+ HDR mode renders everything SDR (which basically means all PC applications including the desktop itself and SDR games) with 100cd/m^2 brightness. Depending on where this brightness level is on your particular HDR screen it may result in SDR content being too dark, washed out or both.
This setting should help HDR users on PC to tweak the default system level of SDR white which in turn should make the desktop, applications and games a lot more usable in Windows 10 HDR mode on a wider number of HDR displays. Those of you who are using Insider builds and have issues with HDR in current Win10 release should check this one out.