Clear SynchronizationContext in ToTask()

UI frameworks (WinForms/WPF/Avalonia) install a SynchronizationContext on the test thread. Without a running message pump, async IO in Verify's pipeline would post continuations to that context and deadlock. Clearing it synchronously in ToTask() before the pipeline starts prevents the capture. Adds a test that reproduces the scenario using a recording context.

Author: SimonCropp

src/Verify.Tests/SynchronizationContextTests.cs

@@ -0,0 +1,49 @@
+public class SynchronizationContextTests
+{
+    // WinForms/WPF/Avalonia install a SynchronizationContext on the current thread when a control is
+    // created. During a test that context has no running message pump, so if Verify's async IO
+    // captured it, the continuation that writes the received file for a new or mismatched snapshot
+    // would be posted to a pump that never runs - deadlocking the pipeline and the awaiting test.
+    // SettingsTask.ToTask clears the ambient context so the pipeline runs free of it. This reproduces
+    // that scenario without a UI framework: it records whether the pipeline ever posts a continuation
+    // to the caller's context (dispatching it onward so the test itself never hangs, even on regression).
+    [Fact]
+    public async Task DoesNotCaptureCallerSynchronizationContext()
+    {
+        var context = new RecordingSynchronizationContext();
+        var original = SynchronizationContext.Current;
+        SynchronizationContext.SetSynchronizationContext(context);
+        Task task;
+        try
+        {
+            var settings = new VerifySettings();
+            settings.DisableDiff();
+            // A binary target with no baseline writes the received file via IoHelpers.WriteStream,
+            // whose CopyToAsync onto an async FileStream suspends - the exact async IO that captured
+            // the UI context and deadlocked. (Strings use a synchronous write path and would not.)
+            task = Verify(new MemoryStream(new byte[1024 * 1024]), "bin", settings).ToTask();
+        }
+        finally
+        {
+            // Restore before awaiting so the test's own continuations are unaffected by the fake context.
+            SynchronizationContext.SetSynchronizationContext(original);
+        }
+
+        await Assert.ThrowsAsync<VerifyException>(() => task);
+
+        Assert.False(
+            context.Posted,
+            "Verify captured the caller's SynchronizationContext; its async IO must run free of it.");
+    }
+
+    class RecordingSynchronizationContext : SynchronizationContext
+    {
+        public volatile bool Posted;
+
+        public override void Post(SendOrPostCallback callback, object? state)
+        {
+            Posted = true;
+            base.Post(callback, state);
+        }
+    }
+}

src/Verify/SettingsTask.cs

@@ -312,8 +312,24 @@ public VerifySettings CurrentSettings
     }
 
     [Pure]
-    public Task<VerifyResult> ToTask() =>
-        task ??= buildTask(CurrentSettings);
+    public Task<VerifyResult> ToTask()
+    {
+        if (task is not null)
+        {
+            return task;
+        }
+
+        // Verify's pipeline performs async IO without ConfigureAwait(false), so it must not run under
+        // a caller-supplied SynchronizationContext. UI frameworks (WinForms/WPF/Avalonia) install one
+        // on the current thread when a control is created; during a test it has no running message
+        // pump, so capturing it would deadlock the pipeline (and the awaiting test) on the received
+        // file write of a new or mismatched snapshot. Clearing it here - synchronously, before the
+        // task is built and before the caller's await captures a context - keeps the pipeline and the
+        // caller's continuation free of any context the caller already had. UI converters additionally
+        // restore the context around rendering, since Show() re-installs one mid-pipeline.
+        SynchronizationContext.SetSynchronizationContext(null);
+        return task = buildTask(CurrentSettings);
+    }
 
     [Pure]
     public ConfiguredTaskAwaitable<VerifyResult> ConfigureAwait(bool continueOnCapturedContext) =>