writer: Defer update, until post first render to avoid flicker.

Change-Id: Ib16f5b345901adc7036a8a90d0f46aa3499d66ae

sw/source/uibase/inc/conttree.hxx

@@ -137,6 +137,7 @@ protected:
                                     SvTreeListEntry*& rpNewParent,
                                     sal_uLong&        rNewChildPos
                                 ) override;
+    virtual void    Paint( vcl::RenderContext& rRenderContext, const Rectangle& rRect ) override;
     virtual void    MouseButtonDown( const MouseEvent& rMEvt ) override;
 
     void            EditEntry( SvTreeListEntry* pEntry, EditEntryMode nMode );

sw/source/uibase/utlui/content.cxx

@@ -2424,10 +2424,18 @@ void SwContentTree::ExecCommand(sal_uInt16 nCmd, bool bModifier)
 
 void    SwContentTree::ShowTree()
 {
-    m_aUpdTimer.Start();
     SvTreeListBox::Show();
 }
 
+void SwContentTree::Paint( vcl::RenderContext& rRenderContext,
+                           const Rectangle& rRect )
+{
+    // Start the update timer on the first paint; avoids
+    // flicker on the first reveal.
+    m_aUpdTimer.Start();
+    SvTreeListBox::Paint( rRenderContext, rRect );
+}
+
 // folded together will not be glidled
 
 void    SwContentTree::HideTree()