hidden panel flicker on slidein show

I need to show a panel on a click event. The panel needs to slide into view on show and it needs to be anchored to the button once shown (the one that has the click event).

The panel config is as follows:

Code:

{
    xtype: 'container',
    autoScroll: true,
    hidden: true,
    floating: true,
    shadow: false,
    listeners: {
        show: function(panel) {
            panel.el.slideIn('r', {
                easing: 'easeOut',
                duration: 500
            })
        }
 }

The panel is shown in the button's click event handler:

Code:

helpPanel.showBy(me, 'tr-br');

It works fine, except there is a quick / small flicker before the panel is shown. A bit like the panel is shown and then it is hidden before sliding into the view again.

How can I avoid that flicker? Is this the right way to implement a slide-in show?

Thank you

hidden panel flicker on slidein show