diff --git a/src/logic/Windows.swift b/src/logic/Windows.swift
index 62d4484b..c1e99c04 100644
--- a/src/logic/Windows.swift
+++ b/src/logic/Windows.swift
@@ -240,24 +240,24 @@ class Windows {
         }
     }
 
+    /// Updates windows "lastFocusOrder" to ensure unique values based on window z-order.
+    /// Windows are ordered by their position in Spaces.windowsInSpaces() results,
+    /// with topmost windows first.
     static func sortByLevel() {
-        var windowLevelMap = [CGWindowID: Int]()
-        for (index, cgWindowId) in Spaces.windowsInSpaces([Spaces.currentSpaceId]).enumerated() {
+        var windowLevelMap = [CGWindowID?: Int]()
+        for (index, cgWindowId) in Spaces.windowsInSpaces(Spaces.visibleSpaces).enumerated() {
             windowLevelMap[cgWindowId] = index
         }
-        var sortedTuples = Windows.list
-                .filter { $0.cgWindowId != nil }
-                .map { (windowLevelMap[$0.cgWindowId!], $0) }
-        sortedTuples.sort(by: {
-            if $0.0 == nil {
-                return false
+        var orderForUnlistedWindows = Int.max
+        for window in list {
+            if let listedOrder = windowLevelMap[window.cgWindowId] {
+                window.lastFocusOrder = listedOrder
+            } else {
+                window.lastFocusOrder = orderForUnlistedWindows
+                orderForUnlistedWindows -= 1
             }
-            if $1.0 == nil {
-                return true
-            }
-            return $0.0! < $1.0!
-        })
-        Windows.list = sortedTuples.map { $0.1 }
+        }
+        list.sort { $0.lastFocusOrder < $1.lastFocusOrder }
     }
 
     static func refreshThumbnailsAsync(_ screen: NSScreen, _ currentIndex: Int) {