macOS: restore hidden popup windows

We need to to explicitly unhide popup windows that were previously
explicitly hidden by applicationWillHide, so that their visibility will
be effectively restored when the application is unhidden (otherwise the
windows are gone forever even though their internal visibility is set to
true).

Change-Id: I4dbd209b07f769cc815851b40c41db0739ca2dc9
Task-number: QTBUG-71014
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoaapplicationdelegate.mm

@@ -86,6 +86,7 @@
 #include <private/qguiapplication_p.h>
 #include "qt_mac_p.h"
 #include <qpa/qwindowsysteminterface.h>
+#include <qwindowdefs.h>
 
 QT_USE_NAMESPACE
 
@@ -93,6 +94,7 @@ QT_USE_NAMESPACE
     bool startedQuit;
     NSObject <NSApplicationDelegate> *reflectionDelegate;
     bool inLaunch;
+    QWindowList hiddenWindows;
 }
 
 + (instancetype)sharedDelegate
@@ -322,12 +324,28 @@ QT_USE_NAMESPACE
     // fact that the application itself is hidden, which will cause a problem when
     // the application is made visible again.
     const QWindowList topLevelWindows = QGuiApplication::topLevelWindows();
-    for (QWindow *topLevelWindow : qAsConst(topLevelWindows)) {
-        if ((topLevelWindow->type() & Qt::Popup) == Qt::Popup && topLevelWindow->isVisible())
+    for (QWindow *topLevelWindow : topLevelWindows) {
+        if ((topLevelWindow->type() & Qt::Popup) == Qt::Popup && topLevelWindow->isVisible()) {
             topLevelWindow->hide();
+
+            if ((topLevelWindow->type() & Qt::Tool) == Qt::Tool)
+                hiddenWindows << topLevelWindow;
+        }
     }
 }
 
+- (void)applicationDidUnhide:(NSNotification *)notification
+{
+    if (reflectionDelegate
+        && [reflectionDelegate respondsToSelector:@selector(applicationDidUnhide:)])
+        [reflectionDelegate applicationDidUnhide:notification];
+
+    for (QWindow *window : qAsConst(hiddenWindows))
+        window->show();
+
+    hiddenWindows.clear();
+}
+
 - (void)applicationDidBecomeActive:(NSNotification *)notification
 {
     if (reflectionDelegate