Watch Victoria Lampard's report for ITV News Anglia.
The family of Polish man killed by a single "Superman punch" had to leave court as jurors were shown CCTV of the attack.
Arkadiusz Jozwik died from head injuries two days after the assault in the Stow shopping area of Harlow in August 2016.
A 16-year-old boy, who was 15 at the time of the attack and cannot be named because of his age, denies manslaughter.
As his trial began at Chelmsford Crown Court, the prosecution set out its case that the youth had intended to cause harm to Mr Jozwik - known as Arek to his friends.
The 40-year-old had been drinking with two friends and had just bought a pizza when he was set upon outside a row of takeaway shops shortly after 11.30pm.
The slightly built boy sat with his parents at the back of the courtroom while Judge Patricia Lynch and barristers sat without their wigs and robes due to his age.
Ms Cottage said Mr Jozwik had been drinking vodka with his friend Radek Koscelski and they were both "speaking loudly and appeared to be drunk and smelt of alcohol".
She said a third friend, Jakub Lusiecki, drank beer and "appeared to be sober".
The three men sat on a bench to eat pizza near a group of teenagers with bikes who had been there for most of the day and the defendant was part of the group, Ms Cottage told the court.
She said the attention of the Polish men was drawn to the group of teenagers as "something was kicked or thrown close to them" in The Stow shopping precinct.
Ms Cottage said Mr Koscelski "was swaying and drunk and may have been argumentative, offering to fight", but he made no physical contact.
She said two boys cycled close to Mr Koscelski and Mr Jozwik, which "seemed to spark a disagreement" between the two groups.
Showing CCTV footage to jurors, Ms Cottage said it appeared Mr Koscelski tripped over his own feet then the youth "appears to jump in the air and swing his arm with force to the head of Mr Jozwik".
Some of Mr Jozwik's family were in court, and some left the room as the CCTV footage was shown.
Blood alcohol tests on Mr Jozwik showed he was around "twice over the legal limit to drive", Ms Cottage said.
The trial continues.