The deleted comment's row height is measured and the row folds vertically by exactly that value while fading, and the comments below compute the vacated distance and spring up to fill it

TealMap8 min ago

This is the first comment. Try deleting it.
SandHill25 min ago

Second comment — the shift grows larger further down.
WaveSound46 min ago

Third comment — watch it settle via spring damping.
MossGarden1 hour ago

Fourth comment — it rises by the full shifted distance.