macOS: Clarify relation between occlusion state, hidden state, and expose

Pick-to: 6.5
Change-Id: I0e05332087fb3f876a9d2fadd9d7dcfd556d5734
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
(cherry picked from commit 7a7aecae2ac7ae411e5e3a719f82cd1a0e7fe5ee)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -371,10 +371,6 @@ void QCocoaWindow::setVisible(bool visible)
         // Make the NSView visible first, before showing the NSWindow (in case of top level windows)
         m_view.hidden = NO;
 
-        // Explicitly mark the view as needing display, as we may
-        // not have drawn anything to the view when it was hidden.
-        [m_view setNeedsDisplay:YES];
-
         if (isContentView()) {
             QWindowSystemInterface::flushWindowSystemEvents(QEventLoop::ExcludeUserInputEvents);
 
@@ -1308,8 +1304,14 @@ void QCocoaWindow::windowDidOrderOffScreen()
 
 void QCocoaWindow::windowDidChangeOcclusionState()
 {
+    // Note, we don't take the view's hiddenOrHasHiddenAncestor state into
+    // account here, but instead leave that up to handleExposeEvent, just
+    // like all the other signals that could potentially change the exposed
+    // state of the window.
     bool visible = m_view.window.occlusionState & NSWindowOcclusionStateVisible;
-    qCDebug(lcQpaWindow) << "QCocoaWindow::windowDidChangeOcclusionState" << window() << "is now" << (visible ? "visible" : "occluded");
+    qCDebug(lcQpaWindow) << "Occlusion state of" << m_view.window << "for"
+        << window() << "changed to" << (visible ? "visible" : "occluded");
+
     if (visible)
         [m_view setNeedsDisplay:YES];
     else

src/plugins/platforms/cocoa/qnsview.mm

@@ -273,15 +273,29 @@ QT_NAMESPACE_ALIAS_OBJC_CLASS(QNSViewMenuHelper);
     return focusWindow;
 }
 
+/*
+    Invoked when the view is hidden, either directly,
+    or in response to an ancestor being hidden.
+*/
 - (void)viewDidHide
 {
+    qCDebug(lcQpaWindow) << "Did hide" << self;
+
     if (!m_platformWindow->isExposed())
         return;
 
     m_platformWindow->handleExposeEvent(QRegion());
+}
 
-    // Note: setNeedsDisplay is automatically called for
+/*
-    // viewDidUnhide so no reason to override it here.
+    Invoked when the view is unhidden, either directly,
+    or in response to an ancestor being unhidden.
+*/
+- (void)viewDidUnhide
+{
+     qCDebug(lcQpaWindow) << "Did unhide" << self;
+
+     [self setNeedsDisplay:YES];
 }
 
 - (BOOL)isTransparentForUserInput