Function joins_group 

pub fn joins_group(
    last: Option<(EditKind, usize)>,
    kind: EditKind,
    at: usize,
) -> bool

Whether an edit continues the group the last one opened, rather than starting an undo step of its own.

Two conditions, both structural: the same kind of edit, landing where the last one left the caret. Deliberately not “within N milliseconds” — a time threshold is a number nobody has measured, and adjacency is what actually distinguishes a run of typing from a fresh thought somewhere else.