I want to add an emphatic AE endorsement of the observations Panj made. To my ear you can't say "just when" without a sense of something "least expected" to follow.
"One can feel happy just when everything is perfect" sounds very strange standing on its own, almost unacceptable. Just as "just as" segues to something it is just as as, "just when" implies an "and then." It's a phrase like "not only," in which case the second shoe to drop would be a "but also."
I'd go as far as to say the sentence will not be understood, by most AE listeners, as meaning "one can feel happy only when everything is perfect." Yes, the isolated word only is identical to one sense of the word just-- but their idiomatic function is not always the same, especially in phrases like "just when." They aren't just interchangeable.
I argued with Panj about how strictly since denotes a specific point in time, citing the phrase "since time immemorial" and others-- but I agree about "just when." It's a form of "just then," and unbreakably linked on the conceptual level.
Incidentally, I was put off from reading this thread right away because the title made no sense-- why the virgule?
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