If those ideas were originally on the agenda because that party wanted them to happen, then logically they should be pleased that they have been adopted. The job of an opposition is to temper the wildest excesses of the party in power. If the party in power adopt what the opposition is promoting, that is a success, not daylight robbery.
Surely a party's aim is to get their preferred legislation through, not to get all the kudos themselves for being the first to put it forward? Are the policies for the benefit of the country or the benefit of a party?
If the important thing is the legislation, then give three cheers and think up some more improvements to suggest. If they grudge the country an improvement because it was put through by someone else, then who is uppermost in their concerns? Looks like it is themselves. Dogs in mangers.
This resentment gives party politics a bad name.