diff --git a/src/api-wrappers/PrivateApis.swift b/src/api-wrappers/PrivateApis.swift
index 2b95ff65..c7edbed1 100644
--- a/src/api-wrappers/PrivateApis.swift
+++ b/src/api-wrappers/PrivateApis.swift
@@ -52,8 +52,61 @@ func CGSGetWindowOwner(_ cid: CGSConnectionID, _ wid: CGWindowID, _ windowCid: i
 func CGSGetConnectionPSN(_ cid: CGSConnectionID, _ psn: inout ProcessSerialNumber) -> CGError
 
 // returns an array of displays (as NSDictionary) -> each having an array of spaces (as NSDictionary) at the "Spaces" key; each having a space ID (as UInt64) at the "id64" key
-// /!\ only returns correct values if the user has checked the checkbox in Preferences > Mission Control > "Displays have separate Spaces"
 // * macOS 10.10+
+// /!\ only returns correct values if the user has checked the checkbox in Preferences > Mission Control > "Displays have separate Spaces"
+// See this example with 2 monitors (1 laptop internal + 1 external):
+// * Output with "Displays have separate Spaces" checked:
+//   [{
+//       "Current Space" =     {
+//           ManagedSpaceID = 4;
+//           id64 = 4;
+//           type = 0;
+//           uuid = "6622AC87-2FD2-48E8-934D-F6EB303AC9BA";
+//       };
+//       "Display Identifier" = "6FBB92D9-84CE-8D20-C114-3B1052DD9529";
+//       Spaces =     (
+//           {
+//               ManagedSpaceID = 4;
+//               id64 = 4;
+//               type = 0;
+//               uuid = "6622AC87-2FD2-48E8-934D-F6EB303AC9BA";
+//           }
+//       );
+//   }, {
+//       "Current Space" =     {
+//           ManagedSpaceID = 5;
+//           id64 = 5;
+//           type = 0;
+//           uuid = "BE05AFA2-B253-4199-B39E-A8E77CD4851B";
+//       };
+//       "Display Identifier" = "BB2327F9-3D4F-FD8F-A0EA-B9745A0B818F";
+//       Spaces =     (
+//           {
+//               ManagedSpaceID = 5;
+//               id64 = 5;
+//               type = 0;
+//               uuid = "BE05AFA2-B253-4199-B39E-A8E77CD4851B";
+//           }
+//       );
+//   }]
+// * Output with "Displays have separate Spaces" unchecked:
+//   [{
+//       "Current Space" =     {
+//           ManagedSpaceID = 4;
+//           id64 = 4;
+//           type = 0;
+//           uuid = "6622AC87-2FD2-48E8-934D-F6EB303AC9BA";
+//       };
+//       "Display Identifier" = Main;
+//       Spaces =     (
+//           {
+//               ManagedSpaceID = 4;
+//               id64 = 4;
+//               type = 0;
+//               uuid = "6622AC87-2FD2-48E8-934D-F6EB303AC9BA";
+//           }
+//       );
+//   }]
 @_silgen_name("CGSCopyManagedDisplaySpaces")
 func CGSCopyManagedDisplaySpaces(_ cid: CGSConnectionID) -> CFArray
 
diff --git a/src/logic/Spaces.swift b/src/logic/Spaces.swift
index ce0d92e5..9b616d85 100644
--- a/src/logic/Spaces.swift
+++ b/src/logic/Spaces.swift
@@ -49,7 +49,10 @@ class Spaces {
         visibleSpaces.removeAll()
         var spaceIndex = SpaceIndex(1)
         (CGSCopyManagedDisplaySpaces(cgsMainConnectionId) as! [NSDictionary]).forEach { (screen: NSDictionary) in
-            let display = screen["Display Identifier"] as! ScreenUuid
+            var display = screen["Display Identifier"] as! ScreenUuid
+            if display as String == "Main", let mainUuid = NSScreen.main?.uuid() {
+                display = mainUuid
+            }
             (screen["Spaces"] as! [NSDictionary]).forEach { (space: NSDictionary) in
                 let spaceId = space["id64"] as! CGSSpaceID
                 idsAndIndexes.append((spaceId, spaceIndex))
diff --git a/src/logic/Window.swift b/src/logic/Window.swift
index eec378e2..4ad81bc8 100644
--- a/src/logic/Window.swift
+++ b/src/logic/Window.swift
@@ -218,8 +218,17 @@ class Window {
     }
 
     func isOnScreen(_ screen: NSScreen) -> Bool {
-        if let screenUuid = screen.uuid(), let screenSpaces = Spaces.screenSpacesMap[screenUuid] {
-            return screenSpaces.contains { $0 == spaceId }
+        if NSScreen.screensHaveSeparateSpaces {
+            if let screenUuid = screen.uuid(), let screenSpaces = Spaces.screenSpacesMap[screenUuid] {
+                return screenSpaces.contains { $0 == spaceId }
+            }
+        } else {
+            if let topLeftCorner = position, let size = size {
+                var screenFrameInQuartzCoordinates = screen.frame
+                screenFrameInQuartzCoordinates.origin.y = NSMaxY(NSScreen.screens[0].frame) - NSMaxY(screen.frame)
+                let windowRect = CGRect(origin: topLeftCorner, size: size)
+                return windowRect.intersects(screenFrameInQuartzCoordinates) && Spaces.visibleSpaces.contains(spaceId)
+            }
         }
         return true
     }