There were many years ago apparently scientific experiments to test the idea that if you left an amphibian in water which you heated up gradually it would not notice, dying when it got too hot. I always thought that a strange idea based on most cruel experiments. Most people think it is untrue. An animal will jump out when it senses the water is getting too hot. I am glad they do, for their sakes.
Some in the political world use the story of the boiling water as an analogy based appropriately on a falsehood to describe the way some people apparently will stick around supporting policies and proposals they dislike intensely if they are introduced slowly and stealthily. There has been a rumour about the Remain forces in the government using this technique to get more Leavers to accept more and more of the EU they are seeking to leave, by gradually introducing these features back into the promised Brexit the government is arranging. Thus we saw a progress of more EU controls, payments and laws being introduced from the original Lancaster House statement to the Florence speech, and from Florence to the Mansion House text, to end up with the Chequers proposals.
Gradually the crucial features of Brexit were eroded or removed. Instead of getting all our money back from Day 1 we were told there would be a big and lingering bill. Instead of getting freedom to set our own benefits and work permit policies, we were told we needed to accept some freedom of movement and some payment of benefits to EU citizens on arrival. Instead of getting full freedom to negotiate our own trade deals, we were told we had to live with accepting many EU rules and regulations which might get in the way of trade agreements. Instead of leaving on 29 March 2019 we were told it would be delayed for another 21 months for no good reason. Instead of getting control of our fish from next year, the timetable slipped and the language implied we would continue to give away much of our fish stock.
As any sensible person would predict, the latest version of Chequers represents unacceptably hot water, so the Brexiteers have indeed jumped out. The government has kindly proved again the commonsense view that you cannot get people to change their minds on fundamental issues by seeking to change them gradually and by stealth. They do notice, just as any animal spots the water getting too hot.