Stretches the pane vertically or horizontally based on docking style.
virtual int StretchPaneDeferWndPos(
The amount, in pixels, to stretch the pane. Use a negative value to shrink the pane.
The actual amount, in pixels, that the pane was stretched.
If necessary, this method modifies nStretchSize to ensure that the pane does not exceed size limits. These limits are obtained by calling CPane::GetAvailableStretchSize and CPane::GetAvailableExpandSize.