diagrams.net, formerly drawio

Current layer does not restrict what you can select.
Current layer only desgnates target layer for pastes and new object creations.

Line endpoints which end at an object are either fixed to a point (indicated by X).
    Green when attaching.
Or floating where system just connects to closest edge of the object‥
    Blue when attaching.

Line waypoints may generally be removed by RMB or menu,
but some line types (orthogonal and curve at least) require a minimum number of waypoints.