SSH: open a shell off the caller's thread, and stop waiting when the caller gives up

tests/DodoSSH.Client.Ssh.Tests/KeyAuthenticationTests.cs

@@ -28,6 +28,44 @@ public sealed class KeyAuthenticationTests(SshServerFixture fixture)
 {
     private static CancellationToken Token => TestContext.Current.CancellationToken;
 
+    /// <remarks>
+    /// Opening a shell is three round trips to the server, and SSH.NET makes them synchronously. They used to
+    /// be made on the caller's thread — the UI thread — behind a <c>Task.FromResult</c>, so the task this
+    /// returned was already finished and the window had stopped painting while it was being made.
+    /// </remarks>
+    [Fact]
+    public async Task OpeningAShell_LeavesTheCallingThreadFree()
+    {
+        await using var connection = await ConnectTrustedAsync(
+            new SshPrivateKeyCredential(Pkcs1(fixture.ClientKey), Passphrase: null));
+
+        var opening = connection.OpenShellAsync(TerminalSize.Default, Token);
+
+        opening.IsCompleted.ShouldBeFalse("three round trips to the server cannot have been made on this thread");
+
+        await using var shell = await opening;
+    }
+
+    /// <remarks>
+    /// The cancellation token used to be read once, before any of those round trips. A caller that gives up
+    /// now stops waiting at once, and the connection is left able to open the next shell.
+    /// </remarks>
+    [Fact]
+    public async Task AShellGivenUpOn_StopsTheWait_AndTheConnectionOpensAnother()
+    {
+        await using var connection = await ConnectTrustedAsync(
+            new SshPrivateKeyCredential(Pkcs1(fixture.ClientKey), Passphrase: null));
+
+        using var giveUp = CancellationTokenSource.CreateLinkedTokenSource(Token);
+
+        var opening = connection.OpenShellAsync(TerminalSize.Default, giveUp.Token);
+        await giveUp.CancelAsync();
+
+        await Should.ThrowAsync<OperationCanceledException>(opening);
+
+        await using var shell = await connection.OpenShellAsync(TerminalSize.Default, Token);
+    }
+
     [Fact]
     public async Task AKeyHeldAsBytes_AuthenticatesAndOpensAShell()
     {