Design mode doesn't look exactly the same as actual?

Hi:

I have been used flex for very long time. I realized there is a major issue that the design mode in flex builder doesn’t look exactly the same as actual. I usually align many panels, datagrid…etc up and down. The design mode has a very good function to magnet controls together. The actual look sometimes are little higher/longer, then it many look bad or even covers the panel/datagrid under it a little bit. I know of course I could use VBox for flex align controls during runtime, but sometimes I just really need x, y positions. Is this a bug or just too difficult for flex to calculate the real height?

What’s your solution?