Well shame on me. Tomorrow never comes. So now it’s time to catch up on the maths for a scrollbar. There are probably many ways of doing this, but this is my favourite because it’s easy, and this example uses a scroll track that is of no significant height. i.e. just a blob or something. Three bits of information are needed. Firstly, the height of the overhang from the movieclip that is being scrolled. So:

overhangHeight = movieclip._height – scrollbar._height;

The next piece of information is how far the scrollbar has been dragged. This is a percentage of the height of the scrollbar.

percentScrolled = (100/scrollbar._height) * distanceScrolledSoFar;

The final piece of information depends on the previous two and tells us the distance the main movieclip has to move to match the position of the scrollbar. I called it offset.

offset = (overhangHeight / 100) * percentScrolled;

So now we have the number we need to move the main movieclip. mc._y = (0 – offset);

Very simple but useful to have it written down, lest I ever have to try and remind myself again.