center slide after changing zoom

Change-Id: I08698b06d0485d2cd3573af665e0621b42d37349

sd/source/ui/inc/ViewShell.hxx

@@ -206,6 +206,7 @@ public:
     virtual void UpdateScrollBars (void);
     void    Scroll(long nX, long nY);
     void    ScrollLines(long nX, long nY);
+    void    ScrollCenter();
     virtual void    SetZoom(long nZoom);
     virtual void    SetZoomRect(const Rectangle& rZoomRect);
     void    InitWindows(const Point& rViewOrigin, const Size& rViewSize,

sd/source/ui/view/viewshe2.cxx

@@ -365,6 +365,33 @@ void ViewShell::SetZoom(long nZoom)
     UpdateScrollBars();
 }
 
+namespace
+{
+    void CenterScrollBar(ScrollBar *pBar)
+    {
+        long nVisSize = pBar->GetVisibleSize();
+        long nMin = pBar->GetRangeMin();
+        long nMax = pBar->GetRangeMax();
+        long nLen = nMax - nMin - nVisSize;
+        long nPos = nMin + nLen/2;
+        pBar->DoScroll(nPos);
+    }
+}
+
+void ViewShell::ScrollCenter()
+{
+    if (mpHorizontalScrollBar.get() != NULL)
+        CenterScrollBar(mpHorizontalScrollBar.get());
+
+    //zoom mode with no panning of the current slide, i.e. the
+    //scrollbar is in change slide mode not pan slide mode
+    if (IsPageFlipMode())
+        return;
+
+    if (mpVerticalScrollBar.get() != NULL)
+        CenterScrollBar(mpVerticalScrollBar.get());
+}
+
 /**
  * Set zoom rectangle for active window. Sets all split windows to the same zoom
  * factor.

sd/source/ui/view/viewshel.cxx

@@ -683,6 +683,7 @@ bool ViewShell::HandleScrollCommand(const CommandEvent& rCEvt, ::sd::Window* pWi
                             nNewZoom = std::min( (long) pWin->GetMaxZoom(), basegfx::zoomtools::zoomIn( nOldZoom ));
 
                         SetZoom( nNewZoom );
+                        ScrollCenter(); //center slide after changing zoom
                         Invalidate( SID_ATTR_ZOOM );
                         Invalidate( SID_ATTR_ZOOMSLIDER );