Add wxScrollWindow::ShouldScrollToChildOnFocus() virtual hook

This method can be overridden to indicate that the scrolled window
doesn't want its children to be scrolled into view when they're focused,
which is the default behaviour.

Also reuse this method for Mac-specific scrollbar workaround.

include/wx/scrolwin.h

@@ -14,6 +14,10 @@
 #include "wx/control.h"
 #include "wx/panel.h"
 
+#ifdef  __WXOSX__
+    #include "wx/scrolbar.h"
+#endif
+
 class WXDLLIMPEXP_FWD_CORE wxScrollHelperEvtHandler;
 class WXDLLIMPEXP_FWD_BASE wxTimer;
 
@@ -305,6 +309,21 @@ protected:
         return size;
     }
 
+    // Can be overridden to return false if the child window shouldn't be
+    // scrolled into view automatically when it gets focus, which is the
+    // default behaviour.
+    virtual bool ShouldScrollToChildOnFocus(wxWindow* child)
+    {
+#if defined(__WXOSX__) && wxUSE_SCROLLBAR
+        if ( wxDynamicCast(child, wxScrollBar) )
+            return false;
+#else
+        wxUnusedVar(child);
+#endif
+
+        return true;
+    }
+
 
     double                m_scaleX;
     double                m_scaleY;

interface/wx/scrolwin.h

@@ -581,6 +581,19 @@ public:
     */
     virtual bool SendAutoScrollEvents(wxScrollWinEvent& event) const;
 
+    /**
+        This method can be overridden in a derived class to prevent scrolling
+        the child window into view automatically when it gets focus.
+
+        The default behaviour is to scroll this window to show its currently
+        focused child automatically, to ensure that the user can interact with
+        it. This is usually helpful, but can be undesirable for some windows,
+        in which case this method can be overridden to return @false for them
+        to prevent any scrolling from taking place when such windows get focus.
+
+        @since 3.1.3
+     */
+    virtual bool ShouldScrollToChildOnFocus(wxWindow* child)
 
 protected:
     /**

src/generic/scrlwing.cpp

@@ -1070,10 +1070,11 @@ void wxScrollHelperBase::HandleOnChildFocus(wxChildFocusEvent& event)
     if ( win == m_targetWindow )
         return; // nothing to do
 
-#if defined( __WXOSX__ ) && wxUSE_SCROLLBAR
-    if (wxDynamicCast(win, wxScrollBar))
+    if ( !ShouldScrollToChildOnFocus(win) )
+    {
+        // the window does not require to be scrolled into view
         return;
-#endif
+    }
 
     // Fixing ticket: https://trac.wxwidgets.org/ticket/9563
     // When a child inside a wxControlContainer receives a focus, the