If? else? getProperty?

Hi. I am using this code to align my main mc to the stage of my flash movie.

Stage.align = "TL";
Stage.scaleMode = "noScale";
main_mc._y = Stage.height/2;
sizeListener = new Object();
sizeListener.onResize = function() {
main_mc._y = Stage.height/2;
};
Stage.addListener(sizeListener);

(this is for a website)
What I want is to detect when main_mc hits the top of the stage/browser window and make it not to go any further as the stage/browser window is made smaller. Did I get myself clear?