Don't wait for a connection to finish when a control is closed

[zadjii-msft]

Summary of the Pull Request

When a pane is closed by a connection, we want to wait until the connection is actually Closed before we fire the actual Closed event. If the connection didn't close gracefully, there are scenarios where we want to print a message to the screen.

However, when a pane is closed by the UI, we don't really care to wait for the connection to be completely closed. We can just do it whenever. So I've moved that call to be on a background thread.

PR Checklist

• Closes Sometimes, tabs delay closing for 2+ seconds #1996
• I work here
• Tests added/passed
• [n/a] Requires documentation to be updated

Detailed Description of the Pull Request / Additional comments

Previously we'd wait for the connection to close synchronously when closing tabs or panes. For misbehaving applications like ssh.exe, that could result in the Close needing to WaitForSingleObject on the UI thread. If the user closed the tab / pane either with a keybinding or with some other UI element, they don't really care to see the error message anymore. They just want the pane closed. So there's no need to wait for the actual connection to close - the app can just continue on with whatever it was doing.

Validation Steps Performed

Messed around with closing tabs, panes, tabs with many panes, the entire window. Did this with keybindings, or by clicking on the 'x' on the tab, the 'x' on the window, or using middle-click.

I'm always scared of things like this, so there's a 50% chance this makes things horribly worse.

[DHowett-MSFT]

Looks great to me.

[ghost]

Hello @DHowett-MSFT!

Because this pull request has the AutoMerge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[DHowett-MSFT]

@msftbot merge this in 1 minute

[ghost]

Hello @DHowett-MSFT!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Thu, 09 Apr 2020 23:27:38 GMT, which is in 1 minute

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".

[ghost]

🎉Windows Terminal Preview v0.11.1121.0 has been released which incorporates this pull request.:tada:

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