var size:Number = sprite.width + MARGIN - frame.margin;

When you set break point in the code and move cursor on the above line FDT only shows the value of size variable. Very useful improvement would be to show all variable values when you roll over on the variable text., so:
when I roll over 'size' it will show the value of size variable
when I roll over 'width ' it will show the value of size variable
when I roll over 'MARGIN ' it will show the value of size MARGIN
when I roll over 'margin' it will show the value of size margin

This feature is present ex. on the Visual Studio and works well, since many times while debugging we want to know what's going on in the code, we also want to know variable values. Sure we can use variables panel but with complex types it takes much time and we loose focus on code just to do the search. On the other hand roll over will take you only half a second to know the variable value, you are staying focused on code and just think how fast you could check few variable/fields values.