I agree, and have been doing this in other apps. Committed, thanks!

In an open mailbox folder arrows indicate the sorting direction of a 
column. However, they also appear in all other columns, although those 
aren't sorted (and therefore cannot have a 'direction'). The arrows 
are confusing for a user, and rather unnecessary, since there actually 
is only one global sorting direction in the underlying code.



I propose to remove the arrows from all columns except from the one 
that is currently active. This has the small disadvantage that you 
cannot change the current sorting direction and column at the same 
time with one click, but I believe the advantages of a cleaner 
interface prevail.



A patch that implements this is attached.