Thanks for your post. This issue has been added to tracker.

example:
I have a file called arnold
and would like to create a gzip file;
I active the gzip commnad and is opened the popup for insert name,

of default there is archive.tgz that highlighted in blue (because is selected)

normally in windows for to insert your name is enough to select archive words and write your name;
but in winscp not is so because how you write the first letter of your name in the textbox is insert automatically other word that you used in the past and this make confusion above all in situation where you duplicate files between directories;
infact if you select a file inner upload e would like duplicate a file from site1 to newsite1, how you select the files and click duplicate, you have this path
/home/admin/public_html/site1/archives/upload/*.*

a simple solution form ake duplicate is
select word site1 and write newsite1, but how you select and write first letter n
you haven't this
/home/admin/public_html/n/archives/upload/*.*
but you can to have
/home/admin/public_html/nerds/*.*
if you have makes in the past other operations with folder nerds;