How do I fix the broken reconnect modal when upgrading a Blazor server app from .NET 9 to .NET 10?

Repro steps

1. Create a new Blazor server app in VS 2026, targeting .NET 9
2. Edit the .csproj file and change the target framework to .NET 10: <TargetFramework>net10.0</TargetFramework>
3. Start the app and make sure you have it open in at least two browser tabs (as one is likely to close automatically when you stop debugging)
4. Stop debugging
5. Wait a moment, then restart the app

Notice that the tab that you had open from step 3 does not reconnect, but instead shows an error in the background (yellow bar at the bottom of the page that says "An unhandled error has occurred") and the reconnect modal has changed from "Reloading" to "Failed to resume the session. Please reload the page."

See this screenshot below.

If you create a new Blazor server app targeting .NET 10, there is a ReconnectModal component, that is used in App.razor, but copying this to the upgraded project and adding the usage doesn't change anything, you still get the results shown in the screenshot.

Anyone any idea how we get the reconnect modal to work in .NET 10?

Here is the screenshot: