It's edging closer to the public launch of Windows 10 Mobile, which is why Microsoft has released a new Preview build to Insiders that aims to improve the stability of the operating system.
As this is a bug-fix-oriented update, there aren't any standout new features of note. However, the Windows 10 Mobile Insider Preview build 10586 does fix issues with Start screen corruption after restoring a backup from a phone with a different resolution, while storage settings now work as intended.
Build 10586 also includes improvements to app download reliability, the Messaging + Skype app, and moving apps to the SD card. Microsoft has also improved the performance of Windows 10 Mobile to the point where users should experience fewer resuming or loading screens while switching between apps.
This build has only been released to Fast Ring users due to an issue with upgrading from build 10581. The bug causes a boot loop at the Windows or operator logo if you performed a factory reset while running the previous build, and although it can be addressed by using a hardware reset key combination, Microsoft strongly advises that users backup their phones before upgrading to build 10586.
The good news is that this build is basically identical to the build shipping on the Lumia 950 and Lumia 950 XL, making it essentially a 'final' version of Windows 10 Mobile. If you're rocking a Windows Phone and want to try out Windows 10 Mobile ahead of time, this would be the build to download and install on your smartphone.