I believe that the recently active threads box only shows threads that have been active in the last 48 hours, so I don't think this is possible.
That seems highly unlikely. Oh! That would account for it being shorter recently! Did you get this belief just from personal inspection?
Yes. I noticed that when the list was short, threads would disappear from it after 48 hours had past. It hasn't occured to me before, as the limiting factor used to be the maximum number of threads (10 and now 20), not the time.
