From what I understand you need to find out when a user clicks on an item and do some calculations from there on.
One way to go is to attach a handler to the MouseDown event of the timeline. In the handler you can perform a check to find out whether an item was clicked. This check should be relatively easy, you need to get the DataContext of the e.OriginalSource. If the data context is a TimelineDataItem, this means an item was clicked and you can get the information you need from it:
sender, MouseButtonEventArgs e)
var source = e.OriginalSource
var item = source.DataContext
var start = item.Start;
Another way to go is to use the selection abilities of the timeline (IsSelectionEnabled, SelectionMode). You can attach a handler of the SelectionChanged and get the information you need from the newly selected item.
Both approaches allow you to find the information you would require about an item. Now you can set the video slider.
The other way is even easier. You need to attach a handler to the event that specifies that the user has changed the position of the video slider. In the handler you need to find the start position. You can then decide what you need to do - one way is to set the visible period start and end properties of the timeline.
Let us know if you need more information.
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