The spinning, bending and writhing of Labour over Brexit are enough to put a professional contortionist to shame. Having been dragged kicking and screaming to a point where Leaver Corbyn will support a confirmatory referendum, a Labour government remains committed to their own Brexit which would be put to the people in a remain versus Labour Brexit referendum. Such a deal is one of those mythical unicorns that are now so associated with the Brexit debate. But let's just suppose that they manage to pull off the impossible and get some kind of negotiated deal that is different to the deal negotiated by Theresa May (though the EU have made it clear there will be no renegotiation). Labour claim they are committed to campaigning to remain. In other words, their current position is to go to the people in a general election, ask them to vote them in to negotiate a different deal which will then be put to the people in a referendum in which Labour's official position is to campaign against their own deal!
It's hardly surprising that Labour's vote has collapsed in opinion polls and electoral contests with leadership like that!