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 52f5e50f11a3ba82e32dc2efc656e4021a3fa4f5

[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>

1 files changed, 5 insertions, 10 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index 9247da814d..da973b5313 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/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)