Blender Git Loki

Make extension mode (shiftkey) mandatory for socket selection. Otherwise the user can end up with unexpectedly selected nodes, since socket selection allows selecting inputs and outputs from different nodes, which in turn also mean the parent nodes are selected. This fits the workflow better, you already needed to press shiftkey before to use the auto-connect operator. It will also solve some problems with the upcoming reroute nodes, where the node is basically a single socket.