Read Disabled when it is asked for rather than at type initialisation

src/DiffEngine.Tests/DisabledTests.cs

@@ -0,0 +1,38 @@
+/// <summary>
+/// <see cref="DiffRunner.Disabled" /> was captured at type initialisation, so a build server or AI
+/// CLI reported after that - which is when a test host reports one, having only just loaded - left
+/// diff tools launching anyway.
+/// </summary>
+[NotInParallel]
+public class DisabledTests
+{
+    [Test]
+    public async Task A_build_server_detected_after_first_use_still_disables()
+    {
+        // As the module initializer left it, and as any consumer that ever set it leaves it
+        DiffRunner.Disabled = false;
+
+        DiffRunner.ResetDisabled();
+        BuildServerDetector.Detected = true;
+
+        await Assert.That(DiffRunner.Disabled).IsTrue();
+    }
+
+    [Test]
+    public async Task Setting_it_pins_it()
+    {
+        DiffRunner.ResetDisabled();
+        BuildServerDetector.Detected = true;
+
+        DiffRunner.Disabled = false;
+
+        await Assert.That(DiffRunner.Disabled).IsFalse();
+    }
+
+    /// <summary>
+    /// Every other test in this assembly runs with it off, which the module initializer does once.
+    /// </summary>
+    [After(Test)]
+    public void Restore() =>
+        DiffRunner.Disabled = false;
+}

src/DiffEngine/DiffRunner.cs

@@ -6,7 +6,33 @@ namespace DiffEngine;
 /// </summary>
 public static partial class DiffRunner
 {
-    public static bool Disabled { get; set; } = DisabledChecker.IsDisable();
+    /// <summary>
+    /// Whether launching a diff tool is turned off, for this process or for this async context.
+    /// <para>
+    /// Read rather than captured, so that the overrides feeding it - <see cref="BuildServerDetector.Detected" />
+    /// and <see cref="AiCliDetector.Detected" />, both of which a test host sets after it has
+    /// loaded - are honoured whenever they are set. Captured once at type initialisation, they
+    /// were inert the moment anything had touched this class, and an AsyncLocal override could
+    /// never have reached a value read into a static anyway.
+    /// </para>
+    /// <para>
+    /// Setting it pins it, and nothing is read from the environment after that.
+    /// </para>
+    /// </summary>
+    public static bool Disabled
+    {
+        get => disabled ?? DisabledChecker.IsDisable();
+        set => disabled = value;
+    }
+
+    static bool? disabled;
+
+    /// <summary>
+    /// Forgets an explicit <see cref="Disabled" />, so it is read from the environment again. For
+    /// tests, which is where anything sets it and then wants the detectors back.
+    /// </summary>
+    internal static void ResetDisabled() =>
+        disabled = null;
 
     public static void MaxInstancesToLaunch(int value) =>
         MaxInstance.SetForAppDomain(value);