Starts a selection at a point: a press (Character: an empty selection at the nearest caret), a double-click (the word there) or a triple-click (the line or paragraph there).
public bool Begin(in PagePoint point, SelectionGranularity granularity = SelectionGranularity.Character)
The point, from TryGetPageAt(in Viewport, Double, Double, out PagePoint).
The unit to select and to extend by.