How to make successive Polygon transitions from A > B, to A > B > C?

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I think you are talking about activating/deactivating vertex/spline points, right?
You can only manipulate vertices (in WorkArea) when they are marked as Active Points. They have a green vertical line on waypoint on the timetrack. If are explicitly deactivated, they have a red vertical line instead.

In the middle of a green-red line mark, the vertex position is controlled by Synfig according to the vertex Origin as shown by Svarov.

Anyway, you can always use the Parameter Panel.

What you may do:

  1. Go back to the time the vertex is still On.
  2. Select it
  3. Check the Parameters Panel. The parameter related to the selected vertex is highlighted there.
  4. Go now to a future time where this vertex must be shown again.
  5. Although the vertex does not appear in the Work Area, the related item in the Parameters Panel is still there - and still highlighted. Right-click on it and select the option “Mark ActivePoint as On”. Attention: the item to be the right-clicked is that named like, for example, “Vertex 002”, and not only “Vertex”.
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