Fix fullscreen new windows opening in current Space (#2345)

* Fix fullscreen new windows opening in current Space

* works

* Stabilize fullscreen tiling regression test

Sources/AppDelegate.swift

@@ -2147,6 +2147,7 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
         let isFirstResponder: Bool
     }
     var debugCloseMainWindowConfirmationHandler: ((NSWindow) -> Bool)?
+    var debugCreateMainWindowSourceIsNativeFullScreenOverride: Bool?
     // Keep debug-only windows alive when tests intentionally inject key mismatches.
     private var debugDetachedContextWindows: [NSWindow] = []
 
@@ -5975,9 +5976,21 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
         // Use the current key window's size for new windows so Cmd+Shift+N
         // creates a window matching the previous one's dimensions.
         let styleMask: NSWindow.StyleMask = [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView]
-        let existingFrame = preferredMainWindowContextForWorkspaceCreation(
+        let sourceContext = preferredMainWindowContextForWorkspaceCreation(
             debugSource: "createMainWindow.initialGeometry"
-        ).flatMap { resolvedWindow(for: $0)?.frame }
+        )
+        let sourceWindow = sourceContext.flatMap { resolvedWindow(for: $0) }
+        let existingFrame = sourceWindow?.frame
+        let sourceWindowIsNativeFullScreen: Bool = {
+#if DEBUG
+            if let debugCreateMainWindowSourceIsNativeFullScreenOverride {
+                return debugCreateMainWindowSourceIsNativeFullScreenOverride
+            }
+#endif
+            return sourceWindow?.styleMask.contains(.fullScreen) == true
+        }()
+        let shouldTemporarilyDisallowFullScreenTiling =
+            sessionWindowSnapshot == nil && sourceWindowIsNativeFullScreen
         let initialRect: NSRect
         if sessionWindowSnapshot == nil, let existingFrame {
             // Convert frame rect to content rect so the new window matches the
@@ -5993,6 +6006,12 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
             backing: .buffered,
             defer: false
         )
+        // When creating a new window from an existing native fullscreen window,
+        // temporarily opt out of fullscreen tiling so AppKit doesn't place the
+        // new window into the active fullscreen Space.
+        if shouldTemporarilyDisallowFullScreenTiling {
+            window.collectionBehavior.insert(.fullScreenDisallowsTiling)
+        }
         window.title = ""
         window.titleVisibility = .hidden
         window.titlebarAppearsTransparent = true
@@ -6045,6 +6064,11 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
             setActiveMainWindow(window)
             NSApp.activate(ignoringOtherApps: true)
         }
+        if shouldTemporarilyDisallowFullScreenTiling {
+            DispatchQueue.main.async { [weak window] in
+                window?.collectionBehavior.remove(.fullScreenDisallowsTiling)
+            }
+        }
         if let restoredFrame {
             window.setFrame(restoredFrame, display: true)
 #if DEBUG

cmuxTests/AppDelegateShortcutRoutingTests.swift

@@ -33,6 +33,7 @@ final class AppDelegateShortcutRoutingTests: XCTestCase {
     override func tearDown() {
         AppDelegate.shared?.shortcutLayoutCharacterProvider = KeyboardLayout.character(forKeyCode:modifierFlags:)
         AppDelegate.shared?.debugCloseMainWindowConfirmationHandler = nil
+        AppDelegate.shared?.debugCreateMainWindowSourceIsNativeFullScreenOverride = nil
         AppDelegate.shared?.dismissNotificationsPopoverIfShown()
         RunLoop.main.run(until: Date(timeIntervalSinceNow: 0.05))
         for action in KeyboardShortcutSettings.Action.allCases {
@@ -98,6 +99,60 @@ final class AppDelegateShortcutRoutingTests: XCTestCase {
         XCTAssertEqual(secondManager.tabs.count, secondCount + 1, "Cmd+N should add workspace to the event's window")
     }
 
+    func testCreateMainWindowDoesNotDisallowFullScreenTilingByDefault() {
+        guard let appDelegate = AppDelegate.shared else {
+            XCTFail("Expected AppDelegate.shared")
+            return
+        }
+
+        let windowId = appDelegate.createMainWindow()
+        defer {
+            closeWindow(withId: windowId)
+        }
+
+        guard let window = window(withId: windowId) else {
+            XCTFail("Expected test window")
+            return
+        }
+
+        XCTAssertFalse(
+            window.collectionBehavior.contains(.fullScreenDisallowsTiling),
+            "Main windows should still support standard macOS Split View when not created from a fullscreen source"
+        )
+    }
+
+    func testCreateMainWindowTemporarilyDisallowsFullScreenTilingFromFullscreenSource() {
+        guard let appDelegate = AppDelegate.shared else {
+            XCTFail("Expected AppDelegate.shared")
+            return
+        }
+
+        appDelegate.debugCreateMainWindowSourceIsNativeFullScreenOverride = true
+
+        let newWindowId = appDelegate.createMainWindow()
+        defer {
+            closeWindow(withId: newWindowId)
+        }
+
+        guard let newWindow = window(withId: newWindowId) else {
+            XCTFail("Expected new window")
+            return
+        }
+
+        XCTAssertTrue(
+            newWindow.collectionBehavior.contains(.fullScreenDisallowsTiling),
+            "New windows should temporarily opt out of fullscreen tiling while opening from a fullscreen source"
+        )
+
+        appDelegate.debugCreateMainWindowSourceIsNativeFullScreenOverride = nil
+        RunLoop.main.run(until: Date(timeIntervalSinceNow: 0.05))
+
+        XCTAssertFalse(
+            newWindow.collectionBehavior.contains(.fullScreenDisallowsTiling),
+            "The fullscreen tiling opt-out should be cleared after initial presentation so Split View keeps working"
+        )
+    }
+
     func testAddWorkspaceInPreferredMainWindowIgnoresStaleTabManagerPointer() {
         guard let appDelegate = AppDelegate.shared else {
             XCTFail("Expected AppDelegate.shared")