QmlDesigner: Fix Qt Advanced Docking System

Fix an issue with a wrong QWidget background rendering when dragging
a dock widget across a floating dock widgets border.

Task-number: QDS-7425
Change-Id: I018cc224a6316a0327cb1806512b6aa6a75d88a3
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/libs/advanceddockingsystem/dockoverlay.cpp

@@ -421,6 +421,7 @@ namespace ADS {
         d->m_lastLocation = InvalidDockWidgetArea;
 
         // Move it over the target.
+        hide();
         resize(target->size());
         QPoint topLeft = target->mapToGlobal(target->rect().topLeft());
         move(topLeft);