Leaning, the 27-year-old former Yorkshire batsman, finished unbeaten on 127 when fists were bumped at 4.50pm and he was joined in a 172-run fourth wicket stand by Ollie Robinson who scored 85 in Kent’s second innings’ 387 for four declared.
Archer, who controversially did not bowl despite fielding on the third evening, when 24 overs only were possible due to earlier rain, appeared in his whites as the teams warmed up but then did not take the field as Kent resumed their second innings on 220 for three, an overnight lead of 109.
Leaning, who also top-scored with a defiant 63 in Kent’s first innings slide to 145 all out, began the day on 61 while Robinson re-started on 11.
Play began at 1.10pm, following heavy early morning rain and the subsequent mopping up, with Archer replaced in the field by substitute fielder Nick Oxley, a Sussex Academy player.
Kent keeper-batsman Robinson looked particularly busy at the crease as he moved steadily to a neatly-played half-century.
To show his intent, Robinson lofted Jack Carson’s off spin straight for four and then swept the slow left arm of Delray Rawlins for another boundary.
Sussex took the second new ball as soon as it became available, with Kent on 258 for three, initially with Carson operating with it down the Hove slope and Ollie Robinson returning for a three-over burst from the Sea End.
Leaning and Robinson held firm, however, with the latter completing his fifty from 85 balls and Leaning, though more circumspect, keeping his score ticking over as the stand reached three figures.
In all, Leaning faced 263 balls, hitting eight fours, while Robinson later pulled a short ball from Rawlins for six and also included six fours in his bright 135-ball knock before being adjudged lbw to Tom Clark’s medium pace despite what looked to be an inside edge. It was Clark’s maiden first-class wicket.
There was no play either between 3pm and 4pm due to more rain, but at least a final mini-session allowed Tawanda Muyeye, 20, to spend time at the crease following his first innings 1 and reach 12 not out on first-class debut.
Afterwards Sussex skipper Ben Brown explained what happened on day three when Archer looked set to bowl but then didn't - and expressed his sympath for the paceman being injured and unable to do what he wants to do.
Brown said: “It was a communication thing with Jofra on Saturday evening. There is no issue. I asked him beforehand if he was going to have a bowl and he said yes in a warm-up capacity. But he was feeling too sore to bowl.
“The issue here is a sad one for Jofra. He’s a champion cricketer, so talented and so gifted, but at the moment he cannot do what he wants to do, he cannot do his skill.
“I really feel for him. He’s in a difficult situation, with that skill of bowling really fast being taken away from him if you like. This week has been very challenging for him. He was hopeful about how it would go, but he’s felt pretty sore in this game.
“Jofra did the right thing playing in the Second XI game the previous week, and we all want him to be fully fit and firing for England and for the Ashes and things like that. We tried to do what was right for Jofra’s career, and he needed to play in this match to prove his fitness, but it just hasn’t worked out this week.”