I’ve added the three default planes and they are used in two situations: if we start a sketch (by clicking the Sketch button or one of the 2D tools buttons) and we have no shapes drawn, the default planes are shown and the planes corresponding to them are added to the tree. The user can select any of them and the sketch is drawn on that plane. The second case is if we want to draw on one of those planes after we have already drawn something else. In this case, we can select any of the planes from the tree and start drawing on it.
There are some bugs I am working on for the sketch creation feature, as well as some improvements for the edit sketch functionality.
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