modalpopupextender event fires but does not show

I lost half a day of my life to this silly mistake today.

I had on my page a panel, with a modal popupextender. When I clicked a link that ran a serverside mdlPopup.show() or a client side $('mdlPopup').show(), the events fired, but no modal popup showed.

The reason for this was because I had set the "visible" attribute of my panel to "false", rather than setting the style of it to "display:none"!

Silly....