test: restore AppDelegate.shared after every XCTest case

cmuxTests/WindowAndDragTests.swift

@@ -587,6 +587,32 @@ final class AppDelegateWindowContextRoutingTests: XCTestCase {
 }
 
 
+/// `AppDelegate.init` installs the new delegate as `AppDelegate.shared`, and
+/// many tests build a throwaway one without putting the host's back. The next
+/// test in the same host then ran against the leftover: detached-inspector
+/// Cmd-W tests failed on main whenever the shard layout placed them after
+/// AppDelegateWindowContextRoutingTests. XCTest runs these two in name order.
+@MainActor
+final class AppDelegateSharedIsolationTests: XCTestCase {
+    private static var sharedBeforeLeak: AppDelegate??
+
+    func test1ConstructingAnAppDelegateReplacesShared() {
+        Self.sharedBeforeLeak = .some(AppDelegate.shared)
+        let leaked = AppDelegate()
+        XCTAssertTrue(AppDelegate.shared === leaked)
+    }
+
+    func test2NextTestStartsWithTheHostSharedDelegate() throws {
+        guard let expected = Self.sharedBeforeLeak else {
+            throw XCTSkip("Runs after test1ConstructingAnAppDelegateReplacesShared in the same host")
+        }
+        XCTAssertTrue(
+            AppDelegate.shared === expected,
+            "A delegate a previous test constructed must not stay installed as AppDelegate.shared"
+        )
+    }
+}
+
 @MainActor
 final class AppDelegateLaunchServicesRegistrationTests: XCTestCase {
     func testDefaultTerminalRegistrationKeepsAllAdvertisedTargets() {