Note that the same issue actually showed without a frame node, when
trying to click-drag on a socket on its 'inside node' part, you would get
same behavior.
Only solution I can see here is to prevent Node selection to go on when
user clicks on one of its sockets, there is no way afaik to make
drag-from-socket to start if we keep select-node operator running
modal, since both operators (NODE_OT_select and NODE_OT_link) use the
same shortcut, if select (which is checked first, being a Tool) returns
modal, then event is considered handled it seems, even though
Passthrough is also returned...
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