xcb: prevent dangling pointer when window focus changes

With the current implementation, the QXcbConnection::m_focusWindow
was holding a dangling pointer during the focus transition from a
dying modal window to other modal window.

1) QXcbConnection::m_focusWindow holds a pointer to A;
2) A is closed;
3) relayFocusToModalWindow B;
   => m_focusWindow now points to a dead window.
4) We get a reply back from WM in a respone to
   relayFocusToModalWindow (_NET_ACTIVE_WINDOW)
   => m_focusWindow now points to a valid window.

The fix is to update m_focusWindow to nullptr, when the current
focus window was destroyed and the new focus window has not been
set yet by WM.

This patch actually solves a more general case - whenever we get a
focus-out event, we should set m_focusWindow to nullptr. It is ok
for none of the windows to be in-focus while focus transition is
happening. The focusInPeeker will make sure to "optimize out" (when possible)
this no-window-in-focus state, and GUI won't be bothered by this
extra event. This is how things were working before relayFocusToModalWindow
was introduced. Having a focus-relay mechanism in-between is ok,
but it should not have changed the original behavior.

Task-number: QTBUG-48391
Task-number: QTBUG-55197
Change-Id: I6fdda9de73f999dad84000059ce4b89c0d1a964c
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.h b/src/plugins/platforms/xcb/qxcbconnection.h
index 01a97a187a..22c7d1aa8e 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.h
+++ b/src/plugins/platforms/xcb/qxcbconnection.h
@@ -480,7 +480,7 @@ public:
     Qt::MouseButton translateMouseButton(xcb_button_t s);
 
     QXcbWindow *focusWindow() const { return m_focusWindow; }
-    void setFocusWindow(QXcbWindow *);
+    void setFocusWindow(QWindow *);
     QXcbWindow *mouseGrabber() const { return m_mouseGrabber; }
     void setMouseGrabber(QXcbWindow *);
     QXcbWindow *mousePressWindow() const { return m_mousePressWindow; }