Make sure the size timeline is inside the Symbol
Then makes sure all the symbol Start Main Timeline action for each symbol instance are removed from the Main Timeline of the Main Scene.
You can then use waypoints/On Enterview Point
Then you can do something like this example.
If using On Enter Viewpoint
I would use an element inside the symbol ( opacity 0 ) which should allow you more adjustment of where the enter point is.
We add @MaxZieb 's hypeDocument.getSymbolInstance extension to the Head so we can easily get the element's parent symbol
You do not use ScrollTop per se to control the timeline, rather you increment the current time in the timeline by 0.1 ( or some number )
i.e symbolInstance.goToTimeInTimelineNamed(lastPoint + .1 , 'Main Timeline');
In this example we also compare the last scrollTop to the current scrollTop.
These are used to try to know if going up or down.
This example is by no means perfect but should give you an idea. ( you probably can look at some of the parallax posts for scroll control but the main take is the symbol setup)
scroll-Symbol_MHv1.hype.zip (1.6 MB)