Handle moving the last surface out of a window

Sources/AppDelegate.swift

@@ -1193,6 +1193,12 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
             }
         }
 
+        cleanupEmptySourceWorkspaceAfterSurfaceMove(
+            sourceWorkspace: sourceWorkspace,
+            sourceManager: source.tabManager,
+            sourceWindowId: source.windowId
+        )
+
         if focus {
             if focusWindow, let destinationWindowId = windowId(for: destinationManager) {
                 _ = focusMainWindow(windowId: destinationWindowId)
@@ -1435,6 +1441,21 @@ final class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCent
         )
     }
 
+    private func cleanupEmptySourceWorkspaceAfterSurfaceMove(
+        sourceWorkspace: Workspace,
+        sourceManager: TabManager,
+        sourceWindowId: UUID
+    ) {
+        guard sourceWorkspace.panels.isEmpty else { return }
+        guard sourceManager.tabs.contains(where: { $0.id == sourceWorkspace.id }) else { return }
+
+        if sourceManager.tabs.count > 1 {
+            sourceManager.closeWorkspace(sourceWorkspace)
+        } else {
+            _ = closeMainWindow(windowId: sourceWindowId)
+        }
+    }
+
     private func windowForMainWindowId(_ windowId: UUID) -> NSWindow? {
         if let ctx = mainWindowContexts.values.first(where: { $0.windowId == windowId }),
            let window = ctx.window {

Sources/Workspace.swift

@@ -2671,9 +2671,16 @@ extension Workspace: BonsplitDelegate {
             lastTerminalConfigInheritancePanelId = nil
         }
 
-        // Keep the workspace invariant: always retain at least one real panel.
-        // This prevents runtime close callbacks from ever collapsing into a tabless workspace.
+        // Keep the workspace invariant for normal close paths.
+        // Detach/move flows intentionally allow a temporary empty workspace so AppDelegate can
+        // prune the source workspace/window after the tab is attached elsewhere.
         if panels.isEmpty {
+            if isDetaching {
+                scheduleTerminalGeometryReconcile()
+                scheduleFocusReconcile()
+                return
+            }
+
             let replacement = createReplacementTerminalPanel()
             if let replacementTabId = surfaceIdFromPanelId(replacement.id),
                let replacementPane = bonsplitController.allPaneIds.first {

cmuxTests/CmuxWebViewKeyEquivalentTests.swift

@@ -2898,6 +2898,34 @@ final class WorkspacePanelGitBranchTests: XCTestCase {
         )
     }
 
+    func testDetachLastSurfaceLeavesWorkspaceTemporarilyEmptyForMoveFlow() {
+        let workspace = Workspace()
+        guard let panelId = workspace.focusedPanelId,
+              let paneId = workspace.paneId(forPanelId: panelId) else {
+            XCTFail("Expected initial panel and pane")
+            return
+        }
+
+        XCTAssertEqual(workspace.panels.count, 1)
+
+        guard let detached = workspace.detachSurface(panelId: panelId) else {
+            XCTFail("Expected detach of last surface to succeed")
+            return
+        }
+
+        XCTAssertEqual(detached.panelId, panelId)
+        XCTAssertTrue(
+            workspace.panels.isEmpty,
+            "Detaching the last surface should not auto-create a replacement panel"
+        )
+        XCTAssertNil(workspace.surfaceIdFromPanelId(panelId))
+        XCTAssertEqual(workspace.bonsplitController.tabs(inPane: paneId).count, 0)
+
+        let restoredPanelId = workspace.attachDetachedSurface(detached, inPane: paneId, focus: false)
+        XCTAssertEqual(restoredPanelId, panelId)
+        XCTAssertEqual(workspace.panels.count, 1)
+    }
+
     func testBrowserSplitWithFocusFalseRecoversFromDelayedStaleSelection() {
         let workspace = Workspace()
         guard let originalFocusedPanelId = workspace.focusedPanelId else {