QDockWidget warning log for an unexpected situation

It didn't enter the original patch because it would
prevent it to be backported to 6.2 and 5.15.

Change-Id: I9af30b86b98d5d101b0784cf45781cf46d216c6d
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
(cherry picked from commit d0505695dd7100412db442824b6b09b7b801fc38)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qdockwidget.cpp

@@ -1002,11 +1002,14 @@ bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
 
             // Calculate new nativePos based on startPos + mouse delta move.
             const QPoint nativeNewPos = nativeWdgOrgPos + (nativeTo - nativeFrom);
-
             pos = QHighDpiScaling::mapPositionFromNative(nativeNewPos, wdgScreen->handle())
                     - windowMarginOffset;
         } else {
             // Fallback in the unlikely case that source and target screens could not be established
+            qCWarning(lcQpaDockWidgets)
+                    << "QDockWidget failed to find relevant screen info. screenFrom:" << screenFrom
+                    << "screenTo:" << screenTo << " wdgScreen:" << wdgScreen << "orgWdgScreen"
+                    << orgWdgScreen;
             pos = event->globalPosition().toPoint() - state->pressPos - windowMarginOffset;
         }