xcb: convert last screen to fake when no screen

available for xrandr 1.5 like we did before for xrandr 1.4
(since a094af0017).

Tested with following combination:

* qtbase -DECM_ENABLE_SANITIZERS=address cmake build
* examples/widgets/widgets/wiggly with
ASAN_OPTIONS=verify_asan_link_order=0 env
* xrandr --output LastScreen --off and --auto.

Fixes: QTBUG-96247
Task-number: QTBUG-42985
Change-Id: Idd95d1a3aa057d23e3adb6635dd1acbb2c853497
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/plugins/platforms/xcb/qxcbconnection_screens.cpp

@@ -378,7 +378,8 @@ void QXcbConnection::initializeScreens(bool initialized)
             }
         }
 
-        qCDebug(lcQpaScreen) << "primary output is" << qAsConst(m_screens).first()->name();
+        if (!m_screens.isEmpty())
+            qCDebug(lcQpaScreen) << "primary output is" << qAsConst(m_screens).first()->name();
     }
 }
 
@@ -499,6 +500,8 @@ void QXcbConnection::initializeScreensFromMonitor(xcb_screen_iterator_t *it, int
         m_virtualDesktops.append(virtualDesktop);
     }
     QList<QPlatformScreen*> old = virtualDesktop->m_screens;
+    if (initialized)
+        m_screens.clear();
 
     QList<QPlatformScreen *> siblings;
 
@@ -523,6 +526,7 @@ void QXcbConnection::initializeScreensFromMonitor(xcb_screen_iterator_t *it, int
                 screen = createScreen_monitor(virtualDesktop, monitor_info, monitors_r->timestamp);
                 QHighDpiScaling::updateHighDpiScaling();
             } else {
+                m_screens << screen;
                 updateScreen_monitor(screen, monitor_info, monitors_r->timestamp);
                 old.removeAll(screen);
             }
@@ -530,14 +534,29 @@ void QXcbConnection::initializeScreensFromMonitor(xcb_screen_iterator_t *it, int
 
         siblings << screen;
 
+        if (primaryScreenNumber() == xcbScreenNumber) {
+            primaryScreen = screen;
+            primaryScreen->setPrimary(true);
+        }
+
         xcb_randr_monitor_info_next(&monitor_iter);
     }
 
     if (siblings.isEmpty()) {
-        // If there are no XRandR outputs or XRandR extension is missing,
-        // then create a fake/legacy screen.
-        auto screen = new QXcbScreen(this, virtualDesktop, nullptr);
-        qCDebug(lcQpaScreen) << "created fake screen" << screen;
+        QXcbScreen *screen = nullptr;
+        if (initialized && !old.isEmpty()) {
+            // If there are no other screens on the same virtual desktop,
+            // then transform the physical screen into a fake screen.
+            screen = static_cast<QXcbScreen *>(old.takeFirst());
+            const QString nameWas = screen->name();
+            screen->setMonitor(nullptr, XCB_NONE);
+            qCDebug(lcQpaScreen) << "transformed" << nameWas << "to fake" << screen;
+        } else {
+            // If there are no XRandR outputs or XRandR extension is missing,
+            // then create a fake/legacy screen.
+            screen = new QXcbScreen(this, virtualDesktop, nullptr);
+            qCDebug(lcQpaScreen) << "create a fake screen: " << screen;
+        }
 
         if (primaryScreenNumber() == xcbScreenNumber) {
             primaryScreen = screen;
@@ -548,8 +567,11 @@ void QXcbConnection::initializeScreensFromMonitor(xcb_screen_iterator_t *it, int
     }
 
     if (initialized) {
-        for (QPlatformScreen *ps : old)
-            destroyScreen(static_cast<QXcbScreen*>(ps));
+        for (QPlatformScreen *ps : old) {
+            virtualDesktop->removeScreen(ps);
+            qCDebug(lcQpaScreen) << "destroy screen: " << ps;
+            QWindowSystemInterface::handleScreenRemoved(ps);
+        }
     }
 
     virtualDesktop->setScreens(std::move(siblings));

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -622,18 +622,28 @@ QXcbScreen::QXcbScreen(QXcbConnection *connection, QXcbVirtualDesktop *virtualDe
 
 void QXcbScreen::setMonitor(xcb_randr_monitor_info_t *monitorInfo, xcb_timestamp_t timestamp)
 {
-    if (!connection()->isAtLeastXRandR15() || !monitorInfo)
+    if (!connection()->isAtLeastXRandR15())
         return;
 
+    m_outputs.clear();
+    m_crtcs.clear();
+
+    if (!monitorInfo) {
+        m_monitor = nullptr;
+        m_output = XCB_NONE;
+        m_crtc = XCB_NONE;
+        m_mode = XCB_NONE;
+        m_outputName = defaultName();
+        // TODO: Send an event to the QScreen instance that the screen changed its name
+        return;
+    }
+
     xcb_randr_select_input(xcb_connection(), screen()->root, true);
 
     m_monitor = monitorInfo;
     QRect monitorGeometry = QRect(m_monitor->x, m_monitor->y,
                                   m_monitor->width, m_monitor->height);
 
-    m_outputs.clear();
-    m_crtcs.clear();
-
     int outputCount = xcb_randr_monitor_info_outputs_length(m_monitor);
     xcb_randr_output_t *outputs = nullptr;
     if (outputCount) {
@@ -702,6 +712,18 @@ void QXcbScreen::setMonitor(xcb_randr_monitor_info_t *monitorInfo, xcb_timestamp
     m_cursor = new QXcbCursor(connection(), this);
 }
 
+QString QXcbScreen::defaultName()
+{
+    QString name;
+    QByteArray displayName = connection()->displayName();
+    int dotPos = displayName.lastIndexOf('.');
+    if (dotPos != -1)
+        displayName.truncate(dotPos);
+    name = QString::fromLocal8Bit(displayName) + QLatin1Char('.')
+            + QString::number(m_virtualDesktop->number());
+    return name;
+}
+
 QXcbScreen::~QXcbScreen()
 {
     delete m_cursor;
@@ -714,12 +736,7 @@ QString QXcbScreen::getOutputName(xcb_randr_get_output_info_reply_t *outputInfo)
         name = QString::fromUtf8((const char*)xcb_randr_get_output_info_name(outputInfo),
                                  xcb_randr_get_output_info_name_length(outputInfo));
     } else {
-        QByteArray displayName = connection()->displayName();
-        int dotPos = displayName.lastIndexOf('.');
-        if (dotPos != -1)
-            displayName.truncate(dotPos);
-        name = QString::fromLocal8Bit(displayName) + QLatin1Char('.')
-                + QString::number(m_virtualDesktop->number());
+        name = defaultName();
     }
     return name;
 }

src/plugins/platforms/xcb/qxcbscreen.h

@@ -196,6 +196,7 @@ public:
                    xcb_randr_get_output_info_reply_t *outputInfo);
     void setCrtc(xcb_randr_crtc_t crtc) { m_crtc = crtc; }
     void setMonitor(xcb_randr_monitor_info_t *monitorInfo, xcb_timestamp_t timestamp = XCB_NONE);
+    QString defaultName();
 
     void windowShown(QXcbWindow *window);
     QString windowManagerName() const { return m_virtualDesktop->windowManagerName(); }