macOS: Determine opaqueness and backgroundColor of NSWindow declaratively

Instead of imperatively trying to keep the logic consistent in many
different call sites.

Task-number: QTBUG-61909
Change-Id: I8d647690c47656f34673555a8a8aa3ec6ffc73d1
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -749,17 +749,13 @@ void QCocoaWindow::setOpacity(qreal level)
         return;
 
     m_view.window.alphaValue = level;
-    m_view.window.opaque = isOpaque();
 }
 
 void QCocoaWindow::setMask(const QRegion &region)
 {
     qCDebug(lcQpaCocoaWindow) << "QCocoaWindow::setMask" << window() << region;
-    if (isContentView())
-        m_view.window.backgroundColor = !region.isEmpty() ? [NSColor clearColor] : nil;
 
     [qnsview_cast(m_view) setMaskRegion:&region];
-    m_view.window.opaque = isOpaque();
 }
 
 bool QCocoaWindow::setKeyboardGrabEnabled(bool grab)
@@ -1318,11 +1314,6 @@ QCocoaNSWindow *QCocoaWindow::createNSWindow(bool shouldBePanel)
     nsWindow.restorable = NO;
     nsWindow.level = windowLevel(flags);
 
-    if (!isOpaque()) {
-        nsWindow.backgroundColor = [NSColor clearColor];
-        nsWindow.opaque = NO;
-    }
-
     if (shouldBePanel) {
         // Qt::Tool windows hide on app deactivation, unless Qt::WA_MacAlwaysShowToolWindow is set
         nsWindow.hidesOnDeactivate = ((type & Qt::Tool) == Qt::Tool) && !alwaysShowToolWindow();

src/plugins/platforms/cocoa/qnswindow.h

@@ -58,6 +58,8 @@ QT_FORWARD_DECLARE_CLASS(QCocoaWindow)
 - (void)sendEvent:(NSEvent*)theEvent;
 - (void)closeAndRelease;
 - (void)dealloc;
+- (BOOL)isOpaque;
+- (NSColor *)backgroundColor;
 @property (nonatomic, readonly) QCocoaWindow *platformWindow;
 @end
 

src/plugins/platforms/cocoa/qnswindow.mm

@@ -153,6 +153,28 @@ static bool isMouseEvent(NSEvent *ev)
     return canBecomeMain;
 }
 
+- (BOOL)isOpaque
+{
+    return self.platformWindow ?
+        self.platformWindow->isOpaque() : qt_objcDynamicSuper();
+}
+
+/*!
+    Borderless windows need a transparent background
+
+    Technically windows with NSTexturedBackgroundWindowMask (such
+    as windows with unified toolbars) need to draw the textured
+    background of the NSWindow, and can't have a transparent
+    background, but as NSBorderlessWindowMask is 0, you can't
+    have a window with NSTexturedBackgroundWindowMask that is
+    also borderless.
+*/
+- (NSColor *)backgroundColor
+{
+    return self.styleMask == NSBorderlessWindowMask
+        ? [NSColor clearColor] : qt_objcDynamicSuper();
+}
+
 - (void)sendEvent:(NSEvent*)theEvent
 {
     // We might get events for a NSWindow after the corresponding platform