Somebody needs to give the Board of Supervisors a time-out. There’s been some bickering and grandstanding ever since the progressives became a 6-5 majority in November, but as The Chronicle’s Emily Green wrote this week, Tuesday’s meeting was a civic meltdown.

There were insults, sniping and shouting. Members called each other hypocrites, accused each other of malicious tactics and dropped collegiality altogether. It reached the point where board clerk Angela Calvillo, generally unobtrusive, felt the need to speak up.

“Madam President,” she said, “if I may just remind the members that pursuant to Rule 5.12 — the conduct of supervisors — you may not impugn the motives of a member of the board.”

She had the unmistakable sound of the only grown-up in the room.

There’s no mystery about what’s going on. The progs have their majority, and they’re wielding it. Supervisor Scott Wiener spoke at the meeting about what he called “the elephant in the room.”

“There is a majority clique on this board,” he said. “And if you are part of that clique, your measure moves forward. If not, your ballot measures get killed.”

To which many may reply: Oh, boo-hoo. That’s how politics works.

But this goes beyond that. This is losing sight of the best interests of the city. Remember, these spats weren’t about whether to make legislation law; they were only to determine if measures should go on the ballot.

Posturing on trees

The flash point was moderate Supervisor Malia Cohen’s measure to rename the Office of Citizen Complaints the Department of Police Accountability, and to broaden its reach to investigate police misconduct. By all accounts, including those of opponents, it was thoughtful, substantial and popular with voters.

But Cohen has been feuding with progressive Supervisor David Campos over his idea to create a public advocate position. The progressives tried to combine Cohen’s popular measure with the controversial public advocate. After lots of wrangling, Cohen’s measure, a sure winner at the ballot box, was killed.

There’s plenty of blame to go around: Cohen lost her temper, Campos has called her actions “corrupt,” and Supervisor Mark Farrell, a moderate, called the tactics “disgusting.”

But to be brutally frank, we’ll survive. Cohen had a good idea, worked hard to put it together, and got caught in partisan politics. Everyone’s mad, but in the big picture it probably doesn’t affect the average San Franciscan that much.

But here’s where this kind of dysfunction hurts the city: trees. Several years ago, the board passed the responsibility for trees on the street to homeowners. Even if they didn’t plant or even ask for a tree in front of their house, residents are not only responsible for trimming the trees, they are also on the hook for repairs if tree roots buckle the sidewalk.

It is one of the most unfair, boneheaded and unreasonable measures on the books. Wiener has been working on this as long as he’s been a supervisor. Now, after hundreds of consultations and public meetings, he has a sensible plan to have the city take over the 105,000 trees on the street, paid for with a relatively small parcel tax. He says the average homeowner would pay $35 a year. Finally, some useful civic action.

Avalos takes a stand

Except that it isn’t. Ever-unpredictable Supervisor Norman Yee came up with his own plan: have the city take over the trees, but with no funding. (What could possibly go wrong?) And Supervisor John Avalos offered another plan to be funded by a carbon tax.

The result was that Wiener had to postpone consideration of his measure because he doesn’t have the votes.

Guys, c’mon. It’s trees. This is a winner. People will like this, vote for this and appreciate it. Stop posturing and do something for the city.

And here’s the really maddening part: They can do it. We’ve seen it happen.

On Tuesday, Mayor Ed Lee’s 0.75 percent sales tax increase looked to be in big trouble. The measure would create $100 million a year for transportation and $50 million a year for homeless services.

Regardless of personal grudges, surely no supervisor would vote against help for transportation and the homeless, would they? They did. On first vote the sales tax went down, 6-5, reflecting the progressive bias against the mayor’s measures.

Wiener requested the vote be rescinded, there was debate, and — of all people — the usually far-left Avalos stepped up. Although he’s been leading the charge against the tax as payback for what he says is a “mean-spirited” encampment measure by Farrell, Avalos announced that he would vote in favor.

Other progressives later joined him, but it was Avalos who turned the tide. It was a principled stand that compromised his stated objectives for the good of the city.

Well done. Let’s see more of that.

C.W. Nevius is a San Francisco Chronicle columnist. His columns appear Tuesday, Thursday and Saturday. Email: cwnevius@sfchronicle.com Twitter: @cwnevius