The public has always cut Liza Minnelli a lot of slack. Her stunning talent, trouper’s work ethic and relentlessly upbeat showwomanship pretty much demanded it. And adorable “openness” about her life, her many loves, trials, failings and burdens can seem refreshing, even if she’s spinning and myth-building with every public moment and pronouncement.

“Growing up Judy Garland’s daughter was not a lotta laughs,” she quips in the new documentary “Liza: A Truly Terrific Absolutely True Story.” And we get it. We take it at face value. She’s earned that.

“Truly Terrific” — it takes its title from a bit of the manic stage patter she’s performed in decades of shows — is an adoring portrait of a Star pushing 80. Writer-director Bruce David Klein, who counts flattering doc portraits of Meat Loaf and Wall Street plunderer Carl Icahn among his credits, got Minnelli to sit for interviews — in which the camera-wise Oscar winning daughter of director Vincente Minnelli amusingly “directs” and lights herself.

And generations of Minnelli’s friends — she and Mia Farrow met as teen Hollywood brats, George Hamilton used to spy little Liza on film sets — and a couple of more objective academics evaluate her talent, personality and challenges and marvel at spunk, her drive, loyalty and the grand life she’s lived despite growing up under the shadow of a Once in a Lifetime Talent, her mercurial and tragic mother, Judy Garland.

“I’m so lucky and I know that,” Minnelli gushes with a modesty that the film undercuts by showing us how hard she worked to deserve that luck.

We learn little about her early life, but we hear all about the first times she threatened to upstage her mother on stage or on TV. Judy wasn’t having it. We pick up on her “quirkiness,” her cultivated “kooky perspective,” see montages of ’70s TV interviews where mostly-forgotten talking heads question her looks (“Ugly?” Really?), her loves and her pedigree — “(tragic, suicidal, drug-abusing) Judy Garland’s daughter.”

The most revealing feature of this upbeat film is trotting out all of the “mentors” that made Minnelli’s look, public persona and created the great showcases for her talent. Kay Thompson to director/choreographer Bob Fosse to fashion designer Halston and on down the line, they “made” the Liza the public embraced and never really abandoned.

The longer-than-long fake eyelashes that popped up in “Caberet,” the Bob Fosse-polished stagecraft and showmanship, the Charles Aznavour-taught way of going “BIG” and putting over a song are delightful Making of Liza details. Little about her persona was an accident. The influence of these mentors is highlighted in chapter headings that lay out Mibnelli’s Rules for Living as she learned them.

“Don’t waste your time with dull people.” “Don’t go around with people you don’t like.”

She learned to “love life” and “how to appreciate everything we’re going through” — dancing around the scoliosis that limited her dancing range but not her ability to master “Fosseisms,” weathering public embarassments over failed love affairs, marriages to gay men, drug addiction and the like.

Her loyalty to the composers Kander & Ebb, the men who made her, is remembered. She “saved” the musical “Chicago” during out of town tryouts by filling in for an ill star, a “star” turn that was more generous than simply canny.

The film is entirely too reliant on friend and musical confidante Michael Feinstein, I thought. Others appreciate, fret, admire, analyze and adore. Feinstein fawns, glosses over everything Minnelli doesn’t really talk about without spinning, and does it so much that one wishes Minelli’s crack at her interviewer Klein, “Gimme a gay break” was aimed at him.

But that’s kind of the way it is with Liza Minnelli. She has been a defiant anachronism, an Old Hollywood, “Born in a trunk” trouper who made her talents matter into the age of disco and beyond.

And we’ve loved her for it, acknowledged her “good genes” and made this “nepo baby” an exemplar of the breed, a meteoric EGOT who made it big by making it ALL big — big energy, big positivity, big gestures, bigger voice.

Rating: unrated, some profanity

Cast: Liza Minnelli, Michael Feinstein, Chita Rivera, Ben Vereen, Lorna Luft, Joel Grey, John Kander, Darren Criss, George Hamilton and Mia Farrow, with Bob Fosse, Judy Garland, Charles Aznavour

Credits: Scripted and directed by Bruce David Klein. A Kino Lorber release.

Running time: 1:44