xcb: do not create a dummy QScreen when there are no outputs

Whenever a QWindow is associated with a QScreen, the screen is expected
to be a real working one, so that rendering continues to be possible.
This partially reverts 52f5e50f11

[ChangeLog][QPA][Xcb] If all QScreens (xcb outputs) are disconnected
while an application is running, QGuiApplication::primaryScreen() will
return null until a screen is connected again.

Task-number: QTBUG-40174
Task-number: QTBUG-42985
Change-Id: Id1b29dd70eaf3f2e7fd477516ce7e2bf24e095f6
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

src/gui/kernel/qguiapplication.cpp

@@ -873,9 +873,14 @@ QWindowList QGuiApplication::topLevelWindows()
 }
 
 /*!
-    Returns the primary (or default) screen of the application.
+    Returns the primary (or default) screen of the application, or null if there is none
 
     This will be the screen where QWindows are initially shown, unless otherwise specified.
+
+    On some platforms, it may be null when there are actually no screens connected.
+    It is not possible to start a new QGuiApplication while there are no screens.
+    Applications which were running at the time the primary screen was removed
+    will stop rendering graphics until one or more screens are restored.
 */
 QScreen *QGuiApplication::primaryScreen()
 {

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -215,16 +215,6 @@ void QXcbConnection::updateScreens()
             }
             free(primary);
         }
-        // If there's no randr extension, or there was some error above, or the screen
-        // doesn't have outputs for some other reason (e.g. on VNC or ssh -X), just assume there is one screen.
-        if (connectedOutputCount == 0) {
-            qCDebug(lcQpaScreen, "found a screen with zero outputs");
-            QXcbScreen *screen = findOrCreateScreen(newScreens, xcbScreenNumber, xcbScreen);
-            siblings << screen;
-            activeScreens << screen;
-            if (!primaryScreen)
-                primaryScreen = screen;
-        }
         foreach (QPlatformScreen* s, siblings)
             ((QXcbScreen*)s)->setVirtualSiblings(siblings);
         xcb_screen_next(&it);
@@ -282,6 +272,11 @@ void QXcbConnection::updateScreens()
 
     if (!m_screens.isEmpty())
         qCDebug(lcQpaScreen) << "primary output is" << m_screens.first()->name();
+    else
+        // QTBUG-40174, QTBUG-42985: If there are no outputs, then there must be
+        // no QScreen instances; a Qt application can survive this situation, and
+        // start rendering again later when there is a screen again.
+        qCDebug(lcQpaScreen) << "xcb connection has no outputs";
 }
 
 QXcbConnection::QXcbConnection(QXcbNativeInterface *nativeInterface, bool canGrabServer, const char *displayName)

src/widgets/kernel/qdesktopwidget.qdoc

@@ -238,6 +238,11 @@
     \property QDesktopWidget::screenCount
     \brief the number of screens currently available on the system.
 
+    Note that on some platforms, screenCount will be zero if there are actually
+    no screens connected. Applications which were running at the time the
+    screenCount went to zero will stop rendering graphics until one or more
+    screens are restored.
+
     \since 4.6
 */