Somewhere on Joel on Software, Joel Spolsky commented that their discussions board adds something like "lastModified=unixEpochTimestampt" to all the top level links. (Actually, what they add is the number of replies, but doing it this way would let us catch edits, too...)

The board's code ignores that parameter, but it lets your browser tell you if a discussion has changes since you read it.

It might not be a bad idea for PM, and it's probably not impossible to implement, I'd think...

The downside is that top-level nodes would either need to know the "most recent modtime" anywhere in their descendants, or would have to do a search every time newest nodes is generated. Obviously, it'd be cheaper to store it, but I don't know how hard it is to modify the db...
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Mike