I am using pacui + yay and diffprog is set to “nvim -d” and never had much troubles merging them.
I am prompted for pacnew only if theres a new entries to that files.
I only merge stuff I understand what they do.
Example,
I uncommented an option somwere to enable advance feature and pacnew file will show it as a change. I know I did that uncommenting so I leave it there.
When it’s a change of syntax or a rewrite of some comments I do merge, because that psychologically helps during the next pacnew merge where there is less to merge.
When a number in a shadow file has changed I strictly avoid merging those because for the love of can’t understand why the packages brings with them that file that if merged gives user a huge troubles.
EDIT: eh, it just happened to another soul Chose "overwrite" old files with .pacnew - now can't login