UI can hang due to not having proper SynchronizationContext for UI interaction

[MarkZuber]

Which Version of MSAL are you using ?
Note that to get help, you need to run the latest version. Preview version are also ok.
For ADAL, please log issues to https://github.com/AzureAD/azure-activedirectory-library-for-dotnet
MSAL 3.0.1-preview

Platform
Windows (net45)

What authentication flow has the issue?

• Desktop / Mobile
  • [ x] Interactive
  • Integrated Windows Auth
  • Username Password
  • Device code flow (browserless)
• Web App
  • Authorization code
  • OBO
• Web API
  • OBO

We check internally when starting the windows web authentication dialog that we're running on an MTA or STA thread. Since we're using async await, we'll usually end up here on an MTA thread and try to create a new STA thread to run the browser on. This doesn't work in some cases when the parent window isn't currently pumping messages (like in VS's case) and so the app hangs with a deadlock. We need to capture the synchronization context at acquiretokeninteractive so that we can use the appropriate sync context when creating the interactive browser dialog.