Error thrown for application after installing latest package to coreapp3.1 project

[cortex-ab]

Bug description

We have a web application that we have just installed the latest version of Microsoft.VisualStudio.Threading onto (17.2.32).
After deploying it to our test environments, an error was thrown:
The framework 'Microsoft.WindowsDesktop.App', version '3.1.0' was not found.
It seems there is now a requirement on that where there wasn't before.

Repro steps

I created an empty project, csproj:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

</Project>

after building it, the runtimeconfig.json file produced looks like this:

{
  "runtimeOptions": {
    "tfm": "netcoreapp3.1",
    "framework": {
      "name": "Microsoft.AspNetCore.App",
      "version": "3.1.0"
    },
    "configProperties": {
      "System.GC.Server": true,
      "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization": false
    }
  }
}

After installing the package:
csproj:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.VisualStudio.Threading" Version="17.2.32" />
  </ItemGroup>

</Project>

runtimeconfig.json:

{
  "runtimeOptions": {
    "tfm": "netcoreapp3.1",
    "frameworks": [
      {
        "name": "Microsoft.WindowsDesktop.App",
        "version": "3.1.0"
      },
      {
        "name": "Microsoft.AspNetCore.App",
        "version": "3.1.0"
      }
    ],
    "configProperties": {
      "System.GC.Server": true,
      "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization": false
    }
  }
}

I'm assuming this was not indended? previous version 17.1.46 does not have the same issue.

I was trying to see what had that dependency and found that

          "PresentationCore": "4.0.0.0",
          "PresentationFramework.Aero": "4.0.0.0",
          "PresentationFramework.Aero2": "4.0.0.0",
          "PresentationFramework.AeroLite": "4.0.0.0",
          "PresentationFramework.Classic": "4.0.0.0",
          "PresentationFramework": "4.0.0.0",
          "PresentationFramework.Luna": "4.0.0.0",
          "PresentationFramework.Royale": "4.0.0.0",
          "PresentationUI": "4.0.0.0",
          "ReachFramework": "4.0.0.0",

was pulled in...but not sure what has referenced them either.

For now I have downgraded the package back to 17.1.46 and everything is working

[AArnott]

This is in fact by design, because for netcoreapp3.1 we added the DispatcherSynchronizationContext-related APIs that previously were only found on net472.
You can avoid the WindowsDesktop runtime requirement by targeting net5.0+, which added the ability for us to split net5.0 from net5.0-windows targeting so that net5.0 consumers could avoid the WindowsDesktop runtime requirement.