macOS: Use native resize cursors for diagonal resize

The diagonal resize cursors provided by Qt does not
quite look like the ones used everywhere else on macOS,
especially for scaled desktops. After some googling this
seems to be how everyone else uses the native versions
since they are private API.

Pick-to: 5.15 6.2 6.3
Change-Id: I3461d07638f4e49cd3e25ab65ef5aa65730bb3a6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoacursor.mm

@@ -47,6 +47,15 @@
 
 #include <QtGui/QBitmap>
 
+#if !defined(QT_APPLE_NO_PRIVATE_APIS)
+@interface NSCursor()
++ (id)_windowResizeNorthWestSouthEastCursor;
++ (id)_windowResizeNorthEastSouthWestCursor;
++ (id)_windowResizeNorthSouthCursor;
++ (id)_windowResizeEastWestCursor;
+@end
+#endif // QT_APPLE_NO_PRIVATE_APIS
+
 QT_BEGIN_NAMESPACE
 
 QCocoaCursor::QCocoaCursor()
@@ -118,7 +127,7 @@ NSCursor *QCocoaCursor::convertCursor(QCursor *cursor)
         return nil;
 
     const Qt::CursorShape newShape = cursor->shape();
-    NSCursor *cocoaCursor;
+    NSCursor *cocoaCursor = nil;
 
     // Check for a suitable built-in NSCursor first:
     switch (newShape) {
@@ -159,7 +168,29 @@ NSCursor *QCocoaCursor::convertCursor(QCursor *cursor)
     case Qt::DragLinkCursor:
         cocoaCursor = [NSCursor dragLinkCursor];
         break;
-    default : {
+#if !defined(QT_APPLE_NO_PRIVATE_APIS)
+    case Qt::SizeVerCursor:
+        if ([NSCursor respondsToSelector:@selector(_windowResizeNorthSouthCursor)])
+            cocoaCursor = [NSCursor _windowResizeNorthSouthCursor];
+        break;
+    case Qt::SizeHorCursor:
+        if ([NSCursor respondsToSelector:@selector(_windowResizeEastWestCursor)])
+            cocoaCursor = [NSCursor _windowResizeEastWestCursor];
+        break;
+    case Qt::SizeBDiagCursor:
+        if ([NSCursor respondsToSelector:@selector(_windowResizeNorthEastSouthWestCursor)])
+            cocoaCursor = [NSCursor _windowResizeNorthEastSouthWestCursor];
+        break;
+    case Qt::SizeFDiagCursor:
+        if ([NSCursor respondsToSelector:@selector(_windowResizeNorthWestSouthEastCursor)])
+            cocoaCursor = [NSCursor _windowResizeNorthWestSouthEastCursor];
+        break;
+#endif // QT_APPLE_NO_PRIVATE_APIS
+    default:
+        break;
+    }
+
+    if (!cocoaCursor) {
         // No suitable OS cursor exist, use cursors provided
         // by Qt for the rest. Check for a cached cursor:
         cocoaCursor = m_cursors.value(newShape);
@@ -174,8 +205,6 @@ NSCursor *QCocoaCursor::convertCursor(QCursor *cursor)
 
             m_cursors.insert(newShape, cocoaCursor);
         }
-
-        break; }
     }
     return cocoaCursor;
 }