QtWidgets: Send show/hide event to children on restore/minimize

Child widgets should get the show/hide event when the TLW changes its
state, because child widgets are also visible or invisible. This
restores the Qt4 behavior (fixes the Qt4->Qt5 regression).

Restoring/minimizing the TLW now sends the spontaneous show/hide event.

Show events are now handled also in the expose event handler in the
QWidgetWindow class, because the show event must occur before the
expose event to avoid possible flicker e.g. the OpenGL content. This
can happen e.g. on XCB platform. If the "WindowStateChange" event occur
before the expose event (e.g. Windows platform) then the code in expose
event handler will be ignored to prevent event duplications.

Added autotest.

Task-number: QTBUG-50589
Change-Id: Ie9a9329b1f29bff876de28d5948d0d5fb6bc1f05
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 15 insertions, 3 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index b1d80d7b8f..b99fca6620 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -276,6 +276,8 @@ QWidgetPrivate::QWidgetPrivate(int version)
       , renderToTextureReallyDirty(1)
       , renderToTextureComposeActive(0)
 #endif
+      , childrenHiddenByWState(0)
+      , childrenShownByExpose(0)
 #if defined(Q_OS_WIN)
       , noPaintOnScreen(0)
 #endif
@@ -9005,13 +9007,23 @@ bool QWidget::event(QEvent *event)
     case QEvent::WindowStateChange: {
         const bool wasMinimized = static_cast<const QWindowStateChangeEvent *>(event)->oldState() & Qt::WindowMinimized;
         if (wasMinimized != isMinimized()) {
+            QWidget *widget = const_cast<QWidget *>(this);
             if (wasMinimized) {
-                QShowEvent showEvent;
-                QCoreApplication::sendEvent(const_cast<QWidget *>(this), &showEvent);
+                // Always send the spontaneous events here, otherwise it can break the application!
+                if (!d->childrenShownByExpose) {
+                    // Show widgets only when they are not yet shown by the expose event
+                    d->showChildren(true);
+                    QShowEvent showEvent;
+                    QCoreApplication::sendSpontaneousEvent(widget, &showEvent);
+                }
+                d->childrenHiddenByWState = false; // Set it always to "false" when window is restored
             } else {
                 QHideEvent hideEvent;
-                QCoreApplication::sendEvent(const_cast<QWidget *>(this), &hideEvent);
+                QCoreApplication::sendSpontaneousEvent(widget, &hideEvent);
+                d->hideChildren(true);
+                d->childrenHiddenByWState = true;
             }
+            d->childrenShownByExpose = false; // Set it always to "false" when window state changes
         }
         changeEvent(event);
     }