The childs of the last canvas did not get snaplines because the canvas is child of a ContentControl which accepts only one child.
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Snaplines were only added for controls when all parent controls (except for root) had a Content property that was a collection.
Consider the structure Window -> Canvas -> UserControl -> Canvas -> Childs.
The childs of the last canvas did not get snaplines because the canvas is child of a ContentControl which accepts only one child.
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