macOS: Make [QNSView wantsLayer] declarative

Change-Id: Ib5dc8178293d13542a54d51484181debd57580f5
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -195,9 +195,6 @@ void QCocoaWindow::initialize()
             [m_view setWantsBestResolutionOpenGLSurface:enable];
             // See also QCocoaGLContext::makeCurrent for software renderer workarounds.
         }
-        BOOL enable = qt_mac_resolveOption(NO, window(), "_q_mac_wantsLayer",
-                                                     "QT_MAC_WANTS_LAYER");
-        [m_view setWantsLayer:enable];
     }
 
     setGeometry(initialGeometry(window(), windowGeometry(), defaultWindowWidth, defaultWindowHeight));

src/plugins/platforms/cocoa/qnsview_drawing.mm

@@ -136,6 +136,18 @@
     }
 }
 
+- (BOOL)wantsLayer
+{
+    Q_ASSERT(m_platformWindow);
+
+    // Toggling the private QWindow property or the environment variable
+    // on and off is not a supported use-case, so this code is effectively
+    // returning a constant for the lifetime of our QSNSView, which means
+    // we don't care about emitting KVO signals for @"wantsLayer".
+    return qt_mac_resolveOption(false, m_platformWindow->window(),
+        "_q_mac_wantsLayer", "QT_MAC_WANTS_LAYER");
+}
+
 - (void)displayLayer:(CALayer *)layer
 {
     Q_ASSERT(layer == self.layer);