Applying loadMask() to a single element in extjs?

How to apply loadMask to only one element, and not the whole browser width-height?

default

here, only one element is masked, and a messageBox is in center, inside this element and not the whole display...

any ideas?

EDIT:

@Molecule , thanks, but this is when data is loading from some source, what i need is :

{
    xtype:"button",
    text:"Alert",
    handler: function(){

        Ext.Msg.show({
        title:'Save Changes?',
        msg: 'You are closing ?',
        buttons: Ext.Msg.YES,
        setLoading: // here somehow only mask parent element, and position alertBox...
    }

}