For each group find first negative value and replace subsequent years with zero

Anonymous  are you getting it from a SQL server? If there is no filter context involved for this task, doing the transformation server side will give you the optimimum performnace.

For example, in TSQL

declare @t1 as table (grp int,cat varchar(10), country int, year int, value int, row int) 
insert into @t1 
select 
  * 
from 
  (
    values 
      (1, 'A', 1, 2018, 50, 1), 
      (1, 'A', 1, 2019, 20, 2), 
      (1, 'A', 1, 2020,-10, 3), 
      (1, 'A', 1, 2021, 20, 4), 
      (1, 'B', 1, 2018, 50, 5), 
      (1, 'B', 1, 2019,-20, 6), 
      (1, 'B', 1, 2020,-10, 7), 
      (1, 'B', 1, 2021, 20, 8)
  ) t(a, b, c, d, e, f);
with cte1 as(
  select 
    grp, cat, country, year, value, row, 
    CASE when sum(value) over ( partition by grp, cat, country, year )< 0 then 0 else null end as _helper 
  from  @t1
) 
select 
  grp, cat, country, year, value, row, 
  CASE WHEN max(_helper) over(partition by grp, cat, country order by year rows unbounded preceding ) IS NULL then value 
      else max(_helper) over (partition by grp, cat, country order by year rows unbounded preceding ) end as Desired 
from cte1 
order by row