Visual Studio Hung after Upgrading <TargetFrameworks>

_This issue has a corresponding [ticket on Developer Community](https://developercommunity.visualstudio.com/t/Visual-Studio-Hung-after-Upgrading/1516461). Please vote and comment there to make sure your voice is heard._

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[severity:It's more difficult to complete my work]
Using this commit:
https://github.com/ExtendedXmlSerializer/home/tree/843c028478368b33d722b645d40457292c1736c4

1. Open solution
2. Install netcoreapp3.1 sdk
3. Attempt to upgrade [this element](https://github.com/ExtendedXmlSerializer/home/blob/843c028478368b33d722b645d40457292c1736c4/test/ExtendedXmlSerializer.Tests.ReportedIssues/ExtendedXmlSerializer.Tests.ReportedIssues.csproj#L4) (and others in the solution) to `netcoreapp3.1`
4. Make sure you save the csproj file :)

Doing so for me ended up in the reported freeze here.

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### Original Comments

#### Feedback Bot on 8/30/2021, 01:53 AM:

(private comment, text removed)
#### Sara Liu [MSFT] on 8/30/2021, 11:52 PM:

(private comment, text removed)
#### Feedback Bot on 12/2/2021, 08:12 PM:

(private comment, text removed)

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### Original Solutions
(no solutions)