Five seems right to me as well, but is there a downside to keeping the preference and just having it default to five?
Well for one, the pref already exists, so changing the default will leave the feature disabled for extant users who have not yet turned it on. This can be worked around of course by changing the pref name.
The other downside is that if a toggle control is added to Preferences > General, it can only toggle the setting between 0 and 5 - erasing the previous setting if it was set to something other than 5.
Both minor points so I don't really care if we go with that route. Giving people the option to change length does have its upsides.
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Aside discussion on hidden preferences:
Hidden preferences do have a place and time. God knows I've personally added enough of them with the quest tracking, familiar trackers, etc.
My change in attitude here derives from the target audience more than anything else. i.e. if we want this change to impact casual users at all, we need to make it opt-out rather than opt-in. Joe Sixpack will simply never find the feature if it is opt-in. That doesn't mean that opt-in features are bad - potentially disruptive features should be off by default. It's a subjective call but I think this feature's potential to be disruptive is low, and the minority of people who find it so can turn it off.
I have more to say but I'm being rushed out the door, so I'll maybe edit this later.