There's never been one case of someone talking, laughing, or doing anything with their heart stopped, let alone when the heart has been shredded by some nasty bade. When the heart stops, everything else does, immediately. Each time I see that scene with Picard laughing with a blade through his chest I wonder what kind of ignorant fool wrote it. In fact, I wonder about the chain of fools that allowed that scene to be part of the final cut.
Not immediately. Nothing is immediate. Picard's laughter takes place a few seconds after the stabbing, before the cardiac arrest has affected his brain cells and as he is falling to the ground, not from pain, but likely in the process of passing out (the first symptom of a sudden cardiac arrest, which is what must be happening).
Here's a couple videos:
Both involved Picard's perception of the event, and we don't know of any visual recording of the incident or third-party reporting, so it's possible that Picard's laughter was more of a bloody gurgle before he passed out and likely lost some memories before restoration. Q is being manipulative here, he doesn't care about an accurate recording of events (and I believe the whole of Tapestry is an illusion).
Picard's laughter, whether real or perceived, is likely an example of the
pseudobulbar effect, with his brain shutting down due to lack of circulation, although it might be too quick given the timeframes shown. The events mentioned in Samaritan Snare ("I had this one Nausicaan down in this somewhat devious joint-lock when, unbeknownst to me, one of his chums drew his weapon and impaled me through the back.") do not match either of the events shown in Tapestry (where he merely tosses one over his shoulder and is stabbed while still standing over him), so false memory seems to be at play here. Either in his story to Wesley or his true conception of the events (if that's what Tapestry is showing). We don't know if he can see the knife, he is likely quickly losing his vision, but his vocal chords and breathing should still be working, if impaired, and I don't see any reason why laughter should be off the table even if his heart was pierced cleanly through.