Fix window snapping when crossing between displays

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

macos/Runner/AppDelegate.swift

@@ -143,16 +143,23 @@ class AppDelegate: FlutterAppDelegate {
       }
       result(nil)
     case "getScreenBounds":
-      // The visible frame of the primary screen (menu bar and dock excluded),
-      // converted to window_manager's top-left origin coordinates.
-      if let screen = NSScreen.screens.first {
-        let f = screen.frame
-        let v = screen.visibleFrame
+      // The union of every screen's visible frame (menu bar and dock
+      // excluded), converted to window_manager's top-left origin coordinates.
+      // The primary screen's full frame is the origin reference, so displays
+      // arranged above it map to negative Y and displays below it to Y beyond
+      // the primary's height — the drag clamp/snap then span the whole desktop.
+      let screens = NSScreen.screens
+      if !screens.isEmpty {
+        let f = screens[0].frame
+        var union = screens[0].visibleFrame
+        for screen in screens.dropFirst() {
+          union = union.union(screen.visibleFrame)
+        }
         result([
-          "x": v.origin.x,
-          "y": f.height - v.origin.y - v.height,
-          "width": v.width,
-          "height": v.height,
+          "x": union.origin.x,
+          "y": f.height - union.origin.y - union.height,
+          "width": union.width,
+          "height": union.height,
         ])
       } else {
         result(nil)