I think most Lamy pens click when posted or capped. I don't have anything with me except my CP1 and it definitely clicks. Pretty sure the Safari does. If you don't mind spending a little more than $50, I highly recommend the Lamy Studio.

You can also get a Rotring Newton for $40 on Ebay. That's one that definitely stays posted.

Another option - Pilot Vanishing Point. You'd be skipping the cap completely. Although that is more like $100.

-Kenny

Is the only one I'm familiar with where the cap positively snaps on when posted. In fact, it won't come off without some effort. However, you can only find this pen used or in a bargain bin, because it's discontinued. Too bad, it was a very nice, lightweight pen with a very smooth Japanese made nib. I bought several when a local Franklin-Covey store was clearancing them out. Wish I'd bought them all!

There are a few pens where the cap screws onto the end of the threaded barrel. Sorry, can't name them off-hand.

The cap of the vintage Parker 51 slips snugly and positively onto the barrel when posted thanks to a spring clutch. Seldom ever slips off.

One of the worst pens I had for posting was a Parker Frontier Flighter (stainless steel). The cap kept slipping off and would fall onto the desk with a big clang. :-) OK pen otherwise.

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"The only reason for time is so that everything doesn't happen at once." Albert Einstein and Buckaroo Banzai

I just got this on from ebay:

Link to ebay listing that will not work forever

The cap positively snaps onto the back of the pen with a click. the pen is also drop deed gorgeous and writes well. It may be a little small and thin for some.

....there was also a click fountain pen--you know click the clicker on the top, and it retracts somehow. I've never seen one, but I thought I read about them. That might be an option, albeit it weird... ;-)

I have a cheapo-Parker, and it has no problems staying on. I also have a hand-made fp that is threaded. I like that a lot, especially the threaded closed part. No wories about the cap falling off at all. :-)

-Jon

and only cost $5
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"I think the surest sign that there is intelligent life out there in the universe is that none of it has tried to contact us." (Calvin and Hobbes/Bill Waterson)

If I understand you correctly about what you mean by click, the Platinum Preppy pens click, seal, and cost 3 bucks from Jetpens.