Open Documents: Always show vertical scrollbar

This stops the 'x' icon from changing position when the scroll bar
vanishes.

Task-number: QTCREATORBUG-16566
Change-Id: I90f35ff34baec0a34e881797cfcc98be383a3f35
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/opendocumentstreeview.cpp

@@ -108,6 +108,7 @@ OpenDocumentsTreeView::OpenDocumentsTreeView(QWidget *parent) :
     setTextElideMode(Qt::ElideMiddle);
     setFrameStyle(QFrame::NoFrame);
     setAttribute(Qt::WA_MacShowFocusRect, false);
+    setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
     viewport()->setAttribute(Qt::WA_Hover);
 
     setSelectionMode(QAbstractItemView::SingleSelection);