Document the Blazor WebEventDescriptor.EventArgsType breaking change (#23000)

* Document the Blazor WebEventDescriptor.EventArgsType breaking change

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

* React to Youssef's feedback

Co-authored-by: Genevieve Warren <[email protected]>

Author: scottaddie

docs/core/compatibility/6.0.md

@@ -1,7 +1,8 @@
 ---
 title: Breaking changes in .NET 6.0
 description: Navigate to the breaking changes in .NET 6.0.
-ms.date: 02/16/2021
+ms.date: 02/24/2021
+no-loc: [Blazor]
 ---
 # Breaking changes in .NET 6.0
 
@@ -15,6 +16,7 @@ If you're migrating an app to .NET 6.0, the breaking changes listed here might a
 - [Nullable reference type annotations changed](aspnet-core/6.0/nullable-reference-type-annotations-changed.md)
 - [Obsoleted and removed APIs](aspnet-core/6.0/obsolete-removed-apis.md)
 - [Blazor: Parameter name changed in RequestImageFileAsync method](aspnet-core/6.0/blazor-parameter-name-changed-in-method.md)
+- [Blazor: WebEventDescriptor.EventArgsType property replaced](aspnet-core/6.0/blazor-eventargstype-property-replaced.md)
 - [Kestrel: Log message attributes changed](aspnet-core/6.0/kestrel-log-message-attributes-changed.md)
 - [Middleware: HTTPS Redirection Middleware throws exception on ambiguous HTTPS ports](aspnet-core/6.0/middleware-ambiguous-https-ports-exception.md)
 - [Razor: RazorEngine APIs marked obsolete](aspnet-core/6.0/razor-engine-apis-obsolete.md)

docs/core/compatibility/aspnet-core/6.0/blazor-eventargstype-property-replaced.md

@@ -0,0 +1,51 @@
+---
+title: "Breaking change: Blazor: WebEventDescriptor.EventArgsType property replaced"
+description: "Learn about the breaking change in ASP.NET Core 6.0 where the WebEventDescriptor.EventArgsType property is replaced by the EventName property."
+author: scottaddie
+ms.author: scaddie
+ms.date: 02/24/2021
+no-loc: [Blazor]
+---
+# Blazor: :::no-loc text="WebEventDescriptor.EventArgsType"::: property replaced
+
+The <xref:Microsoft.AspNetCore.Components.RenderTree.WebEventDescriptor> class is part of Blazor's internal protocol for communicating events from JavaScript into .NET. This class isn't typically used by app code, but rather by platform authors.
+
+Starting in ASP.NET Core 6.0, the <xref:Microsoft.AspNetCore.Components.RenderTree.WebEventDescriptor.EventArgsType%2A> property on `WebEventDescriptor` is being replaced by a new `EventName` property. This change is unlikely to affect any app code, as it's a low-level platform implementation detail.
+
+## Version introduced
+
+6.0
+
+## Old behavior
+
+In ASP.NET Core 5.0 and earlier, the property `EventArgsType` describes a nonstandard, Blazor-specific category name for groups of DOM event types. For example, the `click` and `mousedown` events were both mapped to an `EventArgsType` value of `mouse`. Similarly, `cut`, `copy`, and `paste` events are mapped to an `EventArgsType` value of `clipboard`. These category names are used to determine the .NET type to use for deserializing the incoming event arguments data.
+
+## New behavior
+
+Starting in ASP.NET Core 6.0, the new property `EventName` only specifies the original event's name. For example, `click`, `mousedown`, `cut`, `copy`, or `paste`. There's no longer a need to supply a Blazor-specific category name. For that reason, the old property `EventArgsType` is removed.
+
+## Reason for change
+
+In pull request [dotnet/aspnetcore#29993](https://github.com/dotnet/aspnetcore/pull/29993), support for custom event arguments classes was introduced. As part of this support, the framework no longer relies on all events fitting into a predefined set of categories. The framework now only needs to know the original event name.
+
+## Recommended action
+
+App code should be unaffected and doesn't need to change.
+
+If building a custom Blazor rendering platform, you may need to update the mechanism for dispatching events into the `Renderer`. Replace any hardcoded rules about event categories with simpler logic that supplies the original, raw event name.
+
+## Affected APIs
+
+<xref:Microsoft.AspNetCore.Components.RenderTree.WebEventDescriptor.EventArgsType%2A?displayProperty=nameWithType>
+
+<!--
+
+## Category
+
+ASP.NET Core
+
+## Affected APIs
+
+`P:Microsoft.AspNetCore.Components.RenderTree.WebEventDescriptor.EventArgsType`
+
+-->

docs/core/compatibility/toc.yml

@@ -30,6 +30,8 @@
           href: aspnet-core/6.0/obsolete-removed-apis.md
         - name: "Blazor: Parameter name changed in RequestImageFileAsync method"
           href: aspnet-core/6.0/blazor-parameter-name-changed-in-method.md
+        - name: "Blazor: WebEventDescriptor.EventArgsType property replaced"
+          href: aspnet-core/6.0/blazor-eventargstype-property-replaced.md          
         - name: "Kestrel: Log message attributes changed"
           href: aspnet-core/6.0/kestrel-log-message-attributes-changed.md
         - name: "Middleware: HTTPS Redirection Middleware throws exception on ambiguous HTTPS ports"
@@ -328,6 +330,8 @@
           href: aspnet-core/6.0/obsolete-removed-apis.md
         - name: "Blazor: Parameter name changed in RequestImageFileAsync method"
           href: aspnet-core/6.0/blazor-parameter-name-changed-in-method.md
+        - name: "Blazor: WebEventDescriptor.EventArgsType property replaced"
+          href: aspnet-core/6.0/blazor-eventargstype-property-replaced.md
         - name: "Kestrel: Log message attributes changed"
           href: aspnet-core/6.0/kestrel-log-message-attributes-changed.md
         - name: "Middleware: HTTPS Redirection Middleware throws exception on ambiguous HTTPS ports"