only subtract height of hscroll if visible

otherwise infinite loop in extensions update check

Change-Id: I0dbf4f34208f1bf0854e365006a79470b26571f3

vcl/source/window/layout.cxx

@@ -1706,7 +1706,10 @@ void VclScrolledWindow::setAllocation(const Size &rAllocation)
     // horz. ScrollBar
     if (GetStyle() & WB_AUTOHSCROLL)
     {
-        m_aHScroll.Show(nAvailWidth < aChildReq.Width());
+        bool bShowHScroll = nAvailWidth < aChildReq.Width();
+        m_aHScroll.Show(bShowHScroll);
+
+        if (bShowHScroll)
             nAvailHeight -= getLayoutRequisition(m_aHScroll).Height();
 
         if (GetStyle() & WB_AUTOVSCROLL)