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, 2 insertions, 2 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index 5e600c740b..5e52d4d80f 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection.cpp
@@ -1382,9 +1382,9 @@ void QXcbEventReader::unlock()
     m_mutex.unlock();
 }
 
-void QXcbConnection::setFocusWindow(QXcbWindow *w)
+void QXcbConnection::setFocusWindow(QWindow *w)
 {
-    m_focusWindow = w;
+    m_focusWindow = w ? static_cast<QXcbWindow *>(w->handle()) : nullptr;
 }
 void QXcbConnection::setMouseGrabber(QXcbWindow *w)
 {