Function action_is_document_mutation 

pub fn action_is_document_mutation(action: &Action) -> bool

doesn’t fall through onto the underlying document.

Folding is not here, because folding is not a mutation. A fold changes what is displayed, not what the buffer contains — :help has folds and a magit buffer is almost nothing but folds. They were listed here once, and the reason was never that they write: their handlers mutate self.folds, a slot keyed to document_buffer_id — the buffer BEHIND a focused popup, never swapped for the popup (PIC.2). Grouping them with writes masked that bug instead of fixing it, and the moment this guard widened from two buffer kinds to every read-only buffer, the mask became a regression: <Tab> stopped folding a diff in magit-status, in both renderers. action_mutates_fold_state carries them now, gated on the condition PIC.2 was actually about.