Bleeping Computer has a thorough article (archive.is) on it.
It is an optional update and you have to click on a new "Download and install" option on Windows Update settings page to apply the patch.
It all worked fine for me but the list of "quality improvements" overwhelmed me.
- Addresses an issue that fails to record a local user’s last sign in time even when the user has accessed the server’s network share.
- Addresses an issue that may sever the domain trust relationship when you enable Recycle Bin in the domain that established the trust relationship.
- Addresses an issue that prevents Windows Hello face authentication from working after a restart.
- Updates time zone information for Brazil.
- Addresses an issue to enable Microsoft Edge to print PDF documents that contain landscape and portrait-oriented pages correctly.
- Addresses an issue with PDFs that are configured to be opened only once in Microsoft Edge.
- Addresses an issue that may display colors incorrectly when viewing an image on 10-bit display panels.
- Addresses an issue that may prevent you from changing the display brightness when using certain graphics drivers after resuming from Sleep or Hibernation.
- Addresses an issue in which Windows Graphics Device Interface (GDI+) returns an empty font family name for Bahnschrift.ttf.
- Addresses an issue that may cause a mouse press and release event to sometimes produce an extra mouse move event.
- Addresses an issue that may cause the UI to stop responding for several seconds when scrolling in windows that have many child windows.
- Addresses an issue that fails to bypass automatic sign in (Autologon) when you press and hold the Shift key during startup.
- Addresses an issue that may prevent a device from going to Sleep mode when certain applications that rely on Bluetooth are open.
- Addresses an issue that may reduce Bluetooth audio quality when using certain audio profiles for extended periods.
- Addresses an issue that prevents Microsoft Application Virtualization (App-V) scripting from working if you run it when you’re not connected to a domain controller (DC). App-V scripting also fails when you run it in an environment that only contains Microsoft Azure Active Directory.
- Addresses an issue that causes an error if you open Microsoft OneDrive files on demand when User Experience Virtualization (UE-V) is enabled. To apply this solution, set the following DWORD to 1: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\UEV\Agent\Configuration\ApplyExplorerCompatFix
- Addresses an issue with UE-V that may sometimes prevent exclusion paths from working.
- Addresses an issue that may cause a system that has a Trusted Platform Module (TPM) to stop working.
- Addresses an issue that prevents a system from recognizing a Microsoft account or Azure Active Directory account until the user signs out and signs in again.
- Addresses an issue that may prevent the Netlogon service from establishing a secure channel and reports the error, “0xC000007A – ERROR_PROC_NOT_FOUND.”
- Addresses an issue that fails to update the personal identification number (PIN) policy (minimum length, required digits and special characters, etc.) for Windows Hello for Business when a PIN already exists on the machine.
- Addresses an issue that causes failures to create a recovery drive (USB key) with the error, ”0x80042405[gle=0x00000715].”
- Addresses an issue that prevents an Android emulator based on the virtual machine platform from starting on some systems.
- Addresses an issue that uses a temporary profile to sign in a local user account when the account is configured with a mandatory roaming user profile. The error, “We can’t sign in to your account” appears. The Application event log contains Event 1521, and the event’s source is listed as the Microsoft-Windows-User Profiles Service.
- Addresses an issue that changes the status for Work Folders in File Explorer to 0x80C802A0 (ECS_E_SYNC_UPLOAD_PLACEHOLDER_FAILURE) after selecting Free up space.
- Addresses an issue that may cause a Remote Desktop Server to stop responding when someone who is using drive redirection disconnects.
- Addresses an issue that may cause the Remote Access Connection Manager (RASMAN) service to stop working. You may receive the error “0xc0000005” on devices that have the diagnostic data level manually configured to the non-default setting of 0. You may also receive an error in the Application section of Windows Logs in Event Viewer with Event ID 1000 referencing “svchost.exe_RasMan” and “rasman.dll”. This issue only occurs when a virtual private network (VPN) profile is configured as an Always On VPN (AOVPN) connection with or without a device tunnel.
- Addresses an issue that causes applications on a container host to intermittently lose connectivity because of a port conflict with applications running on a container.
- Addresses an issue that intermittently prevents connections to a corporate network when using Always On VPN with the IKEv2 protocol. Connections are not always automatically established, and manual connections sometimes fail. In this scenario, when you call the RasDial function from the command line for the target VPN connection, you receive the error, “ERROR_PORT_NOT_AVAILABLE(633)”.
- Adds limited support for Windows voice dictation for Chinese Simplified, English (Australia, Canada, India, United Kingdom), French (France), German (Germany), Italian (Italy), Portuguese (Brazil), and Spanish (Mexico, Spain).
- Addresses an issue with opening or using the Window-Eyes screen reader application that may result in an error and prevent some features from functioning as expected.
- Addresses an issue that prevents an App-V application from opening and displays a network failure error. This issue occurs under certain circumstances, such as when a system's battery is low or there is an unexpected power failure.
- Addresses an issue that prevents an App-V application from opening if the client is offline and a startup script is defined for the App-V application.
- Addresses a rare issue that causes Windows Defender Advanced Threat Protection (ATP) to temporarily prevent other processes from accessing files.
- Addresses an issue that causes the Start menu to stop responding when new users sign in to Windows 10, version 1903.
- Updates the Windows Ink Workspace by simplifying the menu and adding direct integration with the Microsoft Whiteboard app for a richer collaboration experience.
Then there's a list of known issues in this update.
- Windows Sandbox may fail to start with "ERROR_FILE_NOT_FOUND (0x80070002)" on devices in which the operating system language is changed during the update process when installing Windows 10, version 1903.
Microsoft is working on a resolution and will provide an update in an upcoming release.
- Devices that start up using Preboot Execution Environment (PXE) images from Windows Deployment Services (WDS) or System Center Configuration Manager (SCCM) may fail to start with the error "Status: 0xc0000001, Info: A required device isn't connected or can't be accessed" after installing this update on a WDS server.
For mitigation instructions, see KB4512816. We are working on a resolution and will provide an update in an upcoming release.
- Devices connected to a domain that is configured to use MIT Kerberos realms may not start up or may continue to restart after installation of this update. Devices that are domain controllers or domain members are both affected.
If you are not sure if your device is affected, contact your administrator. Advanced users can check if this registry key exists HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\Kerberos\MitRealms or for “Define interoperable Kerberos v5 realm settings” policy under Computer Configuration -> Policies -> Administrative Templates > System -> Kerberos.
At this time, we suggest that devices in an affected environment do not install this update. We are working on a resolution and estimate a solution will be available in mid-August.