xcb: Use a placeholder QScreen when there are no outputs connected

If no screens are available, windows could disappear, could stop rendering
graphics, or the application could crash.  This is a real use case in several
scenarios: with x11vnc, when all monitors are physically disconnected from a
desktop machine, or in some cases even when the monitor sleeps.  Now when the
last screen is disconnected, it is transformed into a fake screen.  When a
physical screen appears, the fake QScreen is transformed into a representation
of the physical screen.  Every virtual desktop has its own fake screen, and
primary screens must belong to the primary virtual desktop. It fixes updating
screen geometry on temporarily disabled screens in the middle of the mode
switch.

Expected results: Windows don't disappear, the application doesn't
crash, and QMenu is displayed on the appropriate screen.

This reverts patch 51ada7734ad780178ecced11e0dff454dfc2e5f2

Change-Id: I6e8eb682b0c8425d08ffdaecbd4c6c7700c914b4
Task-number: QTBUG-42985
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.h b/src/plugins/platforms/xcb/qxcbconnection.h
index 3c82170679..fb7cc137b9 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.h
+++ b/src/plugins/platforms/xcb/qxcbconnection.h
@@ -518,15 +518,17 @@ private:
     void initializeXShape();
     void initializeXKB();
     void handleClientMessageEvent(const xcb_client_message_event_t *event);
-    QXcbScreen* createScreen(QXcbVirtualDesktop *virtualDesktop,
-                             xcb_randr_output_t outputId = XCB_NONE,
-                             xcb_randr_get_output_info_reply_t *output = 0);
     QXcbScreen* findScreenForCrtc(xcb_window_t rootWindow, xcb_randr_crtc_t crtc);
     QXcbScreen* findScreenForOutput(xcb_window_t rootWindow, xcb_randr_output_t output);
     QXcbVirtualDesktop* virtualDesktopForRootWindow(xcb_window_t rootWindow);
+    void updateScreens(const xcb_randr_notify_event_t *event);
     bool checkOutputIsPrimary(xcb_window_t rootWindow, xcb_randr_output_t output);
+    void updateScreen(QXcbScreen *screen, const xcb_randr_output_change_t &outputChange);
+    QXcbScreen *createScreen(QXcbVirtualDesktop *virtualDesktop,
+                             const xcb_randr_output_change_t &outputChange,
+                             xcb_randr_get_output_info_reply_t *outputInfo);
+    void destroyScreen(QXcbScreen *screen);
     void initializeScreens();
-    void updateScreens(const xcb_randr_notify_event_t *event);
     bool compressEvent(xcb_generic_event_t *event, int currentIndex, QXcbEventArray *eventqueue) const;
 
     bool m_xi2Enabled;