gistfile1.as

function onStageResize(e:Event) {

// Not pictured: backdrop is a Sprite that has .addChild(new Bitmap()) graphics contained.
// Using Haxe3 + openFL 1.0

if (stage.stageWidth > stage.stageHeight) {
	// This always works:
	backdrop.rotation = 0;
	backdrop.x = 0;
	backdrop.y = 0;
	backdrop.width = stage.stageWidth;
	backdrop.height = stage.stageHeight;
} else {
	backdrop.rotation = 0;
	// The above line should not be necessary. But without it, the following two lines do not take effect:
	backdrop.width = stage.stageHeight;
	backdrop.height = stage.stageWidth;

      	// Probably because the backdrop rotation was triggered on a previous tick.
	backdrop.rotation = 90;

	// (Handle top left registration point:)
	backdrop.x = stage.stageWidth;
	backdrop.y = 0;
}

}