Repro Steps:

1) Create a Page
2) Introduce a Viewbox within the Page's RootGrid.
3) Introduce a Grid within the Viewbox created in Step 2)
4) Equip the Path Tool, and draw a three-Pointed 'V' shape without closing the Path.
5) Attempt to draw a fourth Path Point inside of the 'V' by clicking atop the 'fill' area with the Path Tool.

Observe how the Path Point will not place itself where the mouse cursor is, but instead snaps to the closest Segment of the existing 'V' shape.

6) Remove the Viewbox element, and re-attempt Steps 4) and 5) within a Grid not parented by a Viewbox.

Observe how the Path Point can be placed where the cursor is.

Expected Results: The existence of a Viewbox doesn't have any impact of placing 'interior' Path Points.