Handle gracefully the removal and re-attachment of all QScreens

Don't crash; restore windows when all screens are removed and re-added.
xcb: on configure notify, check for screen change: it may be that a
window belonging to a screen which was removed has now gotten mapped
to the new screen.  On screen change, send a synthetic expose event,
because the real expose events already happened.

Task-number: QTBUG-38326
Task-number: QTBUG-32973
Task-number: QTBUG-42985
Change-Id: If334f55c248468ad3c95e7066bb14eca377d2050
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

src/gui/kernel/qguiapplication.cpp

@@ -2583,6 +2583,8 @@ void QGuiApplicationPrivate::processExposeEvent(QWindowSystemInterfacePrivate::E
         return;
 
     QWindow *window = e->exposed.data();
+    if (!window)
+        return;
     QWindowPrivate *p = qt_window_private(window);
 
     if (!p->receivedExpose) {
@@ -2598,7 +2600,7 @@ void QGuiApplicationPrivate::processExposeEvent(QWindowSystemInterfacePrivate::E
         p->receivedExpose = true;
     }
 
-    p->exposed = e->isExposed;
+    p->exposed = e->isExposed && window->screen();
 
     QExposeEvent exposeEvent(e->region);
     QCoreApplication::sendSpontaneousEvent(window, &exposeEvent);

src/gui/kernel/qplatformwindow.cpp

@@ -83,7 +83,8 @@ QPlatformWindow *QPlatformWindow::parent() const
 */
 QPlatformScreen *QPlatformWindow::screen() const
 {
-    return window()->screen()->handle();
+    QScreen *scr = window()->screen();
+    return scr ? scr->handle() : Q_NULLPTR;
 }
 
 /*!
@@ -479,7 +480,7 @@ QString QPlatformWindow::formatWindowTitle(const QString &title, const QString &
 QPlatformScreen *QPlatformWindow::screenForGeometry(const QRect &newGeometry) const
 {
     QPlatformScreen *currentScreen = screen();
-    if (!parent() && !currentScreen->geometry().intersects(newGeometry)) {
+    if (!parent() && currentScreen && !currentScreen->geometry().intersects(newGeometry)) {
         Q_FOREACH (QPlatformScreen* screen, currentScreen->virtualSiblings()) {
             if (screen->geometry().intersects(newGeometry))
                 return screen;

src/gui/kernel/qscreen.cpp

@@ -86,7 +86,7 @@ QScreen::~QScreen()
     if (this == primaryScreen)
         return;
 
-    bool movingFromVirtualSibling = primaryScreen->handle()->virtualSiblings().contains(handle());
+    bool movingFromVirtualSibling = primaryScreen && primaryScreen->handle()->virtualSiblings().contains(handle());
 
     // Move any leftover windows to the primary screen
     foreach (QWindow *window, QGuiApplication::topLevelWindows()) {

src/gui/kernel/qwindow.cpp

@@ -338,7 +338,7 @@ inline bool QWindowPrivate::windowRecreationRequired(QScreen *newScreen) const
 {
     Q_Q(const QWindow);
     const QScreen *oldScreen = q->screen();
-    return oldScreen != newScreen && platformWindow
+    return oldScreen != newScreen && (platformWindow || !oldScreen)
         && !(oldScreen && oldScreen->virtualSiblings().contains(newScreen));
 }
 
@@ -373,10 +373,13 @@ void QWindowPrivate::setTopLevelScreen(QScreen *newScreen, bool recreate)
     }
     if (newScreen != topLevelScreen) {
         const bool shouldRecreate = recreate && windowRecreationRequired(newScreen);
-        if (shouldRecreate)
+        const bool shouldShow = visibilityOnDestroy && !topLevelScreen;
+        if (shouldRecreate && platformWindow)
             q->destroy();
         connectToScreen(newScreen);
-        if (newScreen && shouldRecreate)
+        if (shouldShow)
+            q->setVisible(true);
+        else if (newScreen && shouldRecreate)
             create(true);
         emitScreenChangedRecursion(newScreen);
     }
@@ -1600,6 +1603,7 @@ void QWindow::destroy()
         QGuiApplicationPrivate::tabletPressTarget = parent();
 
     bool wasVisible = isVisible();
+    d->visibilityOnDestroy = wasVisible && d->platformWindow;
 
     setVisible(false);
 
@@ -1683,7 +1687,7 @@ bool QWindow::setMouseGrabEnabled(bool grab)
 QScreen *QWindow::screen() const
 {
     Q_D(const QWindow);
-    return d->parentWindow ? d->parentWindow->screen() : d->topLevelScreen;
+    return d->parentWindow ? d->parentWindow->screen() : d->topLevelScreen.data();
 }
 
 /*!
@@ -1703,7 +1707,7 @@ void QWindow::setScreen(QScreen *newScreen)
     Q_D(QWindow);
     if (!newScreen)
         newScreen = QGuiApplication::primaryScreen();
-    d->setTopLevelScreen(newScreen, true /* recreate */);
+    d->setTopLevelScreen(newScreen, newScreen != 0);
 }
 
 /*!

src/gui/kernel/qwindow_p.h

@@ -45,6 +45,7 @@
 // We mean it.
 //
 
+#include <QtGui/qscreen.h>
 #include <QtGui/qwindow.h>
 #include <qpa/qplatformwindow.h>
 
@@ -73,6 +74,7 @@ public:
         , parentWindow(0)
         , platformWindow(0)
         , visible(false)
+        , visibilityOnDestroy(false)
         , exposed(false)
         , windowState(Qt::WindowNoState)
         , visibility(QWindow::Hidden)
@@ -147,6 +149,7 @@ public:
     QWindow *parentWindow;
     QPlatformWindow *platformWindow;
     bool visible;
+    bool visibilityOnDestroy;
     bool exposed;
     QSurfaceFormat requestedFormat;
     QString windowTitle;
@@ -175,7 +178,7 @@ public:
     int updateTimer;
 
     QPointer<QWindow> transientParent;
-    QScreen *topLevelScreen;
+    QPointer<QScreen> topLevelScreen;
 
 #ifndef QT_NO_CURSOR
     QCursor cursor;

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -332,6 +332,9 @@ QXcbConnection::QXcbConnection(QXcbNativeInterface *nativeInterface, bool canGra
     initializeXRandr();
     updateScreens();
 
+    if (m_screens.isEmpty())
+        qFatal("QXcbConnection: no screens available");
+
     initializeXFixes();
     initializeXRender();
     m_xi2Enabled = false;

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -249,8 +249,13 @@ void *QXcbNativeInterface::nativeResourceForContext(const QByteArray &resourceSt
 
 void *QXcbNativeInterface::nativeResourceForScreen(const QByteArray &resourceString, QScreen *screen)
 {
+    if (!screen) {
+        qWarning() << "nativeResourceForScreen: null screen";
+        return Q_NULLPTR;
+    }
+
     QByteArray lowerCaseResource = resourceString.toLower();
-    void *result = handlerNativeResourceForScreen(lowerCaseResource ,screen);
+    void *result = handlerNativeResourceForScreen(lowerCaseResource, screen);
     if (result)
         return result;
 

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -417,6 +417,13 @@ void QXcbScreen::handleScreenChange(xcb_randr_screen_change_notify_event_t *chan
 
     QDpi ldpi = logicalDpi();
     QWindowSystemInterface::handleScreenLogicalDotsPerInchChange(QPlatformScreen::screen(), ldpi.first, ldpi.second);
+
+    // Windows which had null screens have already had expose events by now.
+    // They need to be told the screen is back, it's OK to render.
+    foreach (QWindow *window, QGuiApplication::topLevelWindows()) {
+        QXcbWindow *xcbWin = static_cast<QXcbWindow*>(window->handle());
+        xcbWin->maybeSetScreen(this);
+    }
 }
 
 void QXcbScreen::updateGeometry(xcb_timestamp_t timestamp)

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -556,6 +556,14 @@ void QXcbWindow::destroy()
         m_pendingSyncRequest->invalidate();
 }
 
+void QXcbWindow::maybeSetScreen(QXcbScreen *screen)
+{
+    if (!window()->screen() && screen->geometry().contains(geometry().topLeft() * int(devicePixelRatio()))) {
+        QWindowSystemInterface::handleWindowScreenChanged(window(), static_cast<QPlatformScreen *>(screen)->screen());
+        QWindowSystemInterface::handleExposeEvent(window(), QRegion(QRect(QPoint(0, 0), window()->size())));
+    }
+}
+
 void QXcbWindow::setGeometry(const QRect &rect)
 {
     QPlatformWindow::setGeometry(rect);
@@ -737,13 +745,15 @@ void QXcbWindow::hide()
     Q_XCB_CALL(xcb_unmap_window(xcb_connection(), m_window));
 
     // send synthetic UnmapNotify event according to icccm 4.1.4
-    xcb_unmap_notify_event_t event;
-    event.response_type = XCB_UNMAP_NOTIFY;
-    event.event = xcbScreen()->root();
-    event.window = m_window;
-    event.from_configure = false;
-    Q_XCB_CALL(xcb_send_event(xcb_connection(), false, xcbScreen()->root(),
-                              XCB_EVENT_MASK_SUBSTRUCTURE_NOTIFY | XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT, (const char *)&event));
+    if (xcbScreen()) {
+        xcb_unmap_notify_event_t event;
+        event.response_type = XCB_UNMAP_NOTIFY;
+        event.event = xcbScreen()->root();
+        event.window = m_window;
+        event.from_configure = false;
+        Q_XCB_CALL(xcb_send_event(xcb_connection(), false, xcbScreen()->root(),
+                                  XCB_EVENT_MASK_SUBSTRUCTURE_NOTIFY | XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT, (const char *)&event));
+    }
 
     xcb_flush(xcb_connection());
 
@@ -1794,7 +1804,9 @@ void QXcbWindow::handleClientMessageEvent(const xcb_client_message_event_t *even
 QPlatformScreen *QXcbWindow::screenForNativeGeometry(const QRect &newGeometry) const
 {
     QXcbScreen *currentScreen = static_cast<QXcbScreen*>(screen());
-    if (!parent() && !currentScreen->nativeGeometry().intersects(newGeometry)) {
+    if (!currentScreen && QGuiApplication::primaryScreen())
+        currentScreen = static_cast<QXcbScreen*>(QGuiApplication::primaryScreen()->handle());
+    if (currentScreen && !parent() && !currentScreen->nativeGeometry().intersects(newGeometry)) {
         Q_FOREACH (QPlatformScreen* screen, currentScreen->virtualSiblings()) {
             if (static_cast<QXcbScreen*>(screen)->nativeGeometry().intersects(newGeometry))
                 return screen;
@@ -1807,7 +1819,7 @@ void QXcbWindow::handleConfigureNotifyEvent(const xcb_configure_notify_event_t *
 {
     bool fromSendEvent = (event->response_type & 0x80);
     QPoint pos(event->x, event->y);
-    if (!parent() && !fromSendEvent) {
+    if (!parent() && !fromSendEvent && xcbScreen()) {
         // Do not trust the position, query it instead.
         xcb_translate_coordinates_cookie_t cookie = xcb_translate_coordinates(xcb_connection(), xcb_window(),
                                                                               xcbScreen()->root(), 0, 0);
@@ -1825,10 +1837,10 @@ void QXcbWindow::handleConfigureNotifyEvent(const xcb_configure_notify_event_t *
 
     QPlatformWindow::setGeometry(rect);
     QWindowSystemInterface::handleGeometryChange(window(), rect);
-
     QPlatformScreen *newScreen = screenForNativeGeometry(nativeRect);
     if (newScreen != screen()) {
-        QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
+        if (newScreen)
+            QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
         int newDpr = devicePixelRatio();
         if (newDpr != dpr) {
             QRect newRect = mapGeometryFromNative(nativeRect, newDpr);
@@ -2116,7 +2128,7 @@ void QXcbWindow::handlePropertyNotifyEvent(const xcb_property_notify_event_t *ev
             m_windowState = newState;
         }
         return;
-    } else if (event->atom == atom(QXcbAtom::_NET_WORKAREA) && event->window == xcbScreen()->root()) {
+    } else if (event->atom == atom(QXcbAtom::_NET_WORKAREA) && xcbScreen() && event->window == xcbScreen()->root()) {
         xcbScreen()->updateGeometry(event->time);
     }
 }

src/plugins/platforms/xcb/qxcbwindow.h

@@ -153,6 +153,7 @@ public:
 
     virtual void create();
     virtual void destroy();
+    void maybeSetScreen(QXcbScreen *screen);
     QPlatformScreen *screenForNativeGeometry(const QRect &newGeometry) const;
 
 public Q_SLOTS:

src/widgets/kernel/qwidget.cpp

@@ -5536,7 +5536,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
             }
 
             // Native widgets need to be marked dirty on screen so painting will be done in correct context
-            if (backingStore && !onScreen && !asRoot && (q->internalWinId() || !q->nativeParentWidget()->isWindow()))
+            if (backingStore && !onScreen && !asRoot && (q->internalWinId() || (q->nativeParentWidget() && !q->nativeParentWidget()->isWindow())))
                 backingStore->markDirtyOnScreen(toBePainted, q, offset);
 
             //restore

src/widgets/kernel/qwidgetwindow.cpp

@@ -635,7 +635,8 @@ void QWidgetWindow::handleScreenChange()
     sendScreenChangeRecursively(m_widget);
 
     // Invalidate the backing store buffer and repaint immediately.
-    repaintWindow();
+    if (screen())
+        repaintWindow();
 }
 
 void QWidgetWindow::repaintWindow()