Change wxMSW wxTreeCtrl::DoFreeze() to not hide the tree any more.

Hiding the tree when it's frozen, as done in r72665, results in its own
problems, e.g. loss of focus. So don't do this but resize the control to a
very small size when freezing it and restore it to its old size afterwards.

Closes #15166.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74072 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/treectrl.h

@@ -216,6 +216,10 @@ protected:
     virtual void DoFreeze();
     virtual void DoThaw();
 
+    virtual void DoSetSize(int x, int y,
+                           int width, int height,
+                           int sizeFlags = wxSIZE_AUTO);
+
     // SetImageList helper
     void SetAnyImageList(wxImageList *imageList, int which);
 
@@ -336,6 +340,9 @@ private:
     // whether we need to deselect other items on mouse up
     bool m_mouseUpDeselect;
 
+    // The size to restore the control to when it is thawed, see DoThaw().
+    wxSize m_thawnSize;
+
     friend class wxTreeItemIndirectData;
     friend class wxTreeSortHelper;