If you go to /addons/purchase/ there is a file called stock_view.xml and look for the code below in bold.

        <record id="purchase_order_2_stock_picking" model="ir.ui.view">
            <field name="name">Purchase Picking Inherited</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                 <xpath expr="//div[contains(@class, 'oe_title')]" position="before">
                    <div class="oe_right oe_button_box" name="buttons">
                        <button type="object"
                            name="view_picking"
                            string="Incoming Shipments" states="approved"/>
                        <button type="object"  name="invoice_open"
                            string="Invoices" attrs="{'invisible': [('state', '=', 'draft')]}"/> 
                    </div>
                </xpath>
            </field>
        </record>

You can inherit this view and change the states in which it is shown.