Engine: undo groups make several commands one undo step

Session::grouped(label, f) runs f as one undo step on the active
document. On success the group leaves exactly one undo entry holding the
workbook from before the group; on error it restores the workbook,
selection, undo and redo stacks as they were and returns the error.
group_begin, group_end and group_rollback do the same across separate
calls, for an action that runs one step per call.

While a group is open on a document, undoable commands on it add no
undo steps of their own, so a long group neither grows the history nor
pushes older entries past the undo limit, and Undo and Redo are
disabled there. Groups nest: the outermost one makes the step, and a
failing inner group rolls back only its own changes. Closing a document
drops its groups, and a command that panics drops the groups it opened.
history.list reports whether a group is open.

Insert Copied Cells folds its nested steps into one undo step by
counting revisions; inside a group the nested steps push no entries, so
it now leaves the history alone there instead of removing entries from
before the group.

crates/engine/src/cmd/inspect.rs

@@ -11,7 +11,7 @@ pub fn specs() -> Vec<CommandSpec> {
         cmd!(query "sheet.read", "Read Range", [], None, "{range?: \"A1:D10\" (default: used range), sheet?, formulas?: bool, formatted?: bool} → rows of values", has_doc, read),
         cmd!(query "cell.get", "Get Cell", [], None, "{cell?, sheet?} → value, formula, display text, style, comment, link", has_doc, cell_get),
         cmd!(query "selection.stats", "Selection Statistics", [], None, "{} → count, sum, average, min, max", has_doc, sel_stats),
-        cmd!(query "history.list", "Undo History", [], None, "{} → undo/redo labels", has_doc, history),
+        cmd!(query "history.list", "Undo History", [], None, "{} → undo/redo labels, grouping (an undo group is open)", has_doc, history),
         cmd!(query "app.commands", "List Commands", [], None, "{search?} → ids, labels, params", always, list_commands),
     ]
 }
@@ -148,9 +148,11 @@ fn sel_stats(s: &mut Session, _: &Json) -> Result<Json> {
 
 fn history(s: &mut Session, _: &Json) -> Result<Json> {
     let d = s.doc()?;
-    Ok(
-        json!({"undo": d.undo.iter().rev().map(|e| e.label.clone()).collect::<Vec<_>>(), "redo": d.redo.iter().rev().map(|e| e.label.clone()).collect::<Vec<_>>()}),
-    )
+    Ok(json!({
+        "undo": d.undo.iter().rev().map(|e| e.label.clone()).collect::<Vec<_>>(),
+        "redo": d.redo.iter().rev().map(|e| e.label.clone()).collect::<Vec<_>>(),
+        "grouping": s.in_group(d.uid),
+    }))
 }
 
 fn list_commands(s: &mut Session, p: &Json) -> Result<Json> {

crates/engine/src/cmd/mod.rs

@@ -96,12 +96,19 @@ pub fn has_doc(s: &Session) -> std::result::Result<(), String> {
 }
 pub fn can_undo(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
+    not_grouping(s)?;
     if s.active().is_some_and(|d| !d.undo.is_empty()) { Ok(()) } else { Err("nothing to undo".into()) }
 }
 pub fn can_redo(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
+    not_grouping(s)?;
     if s.active().is_some_and(|d| !d.redo.is_empty()) { Ok(()) } else { Err("nothing to redo".into()) }
 }
+/// Undo and Redo wait while an undo group is open on the active document (an action that spans
+/// several calls is still running).
+fn not_grouping(s: &Session) -> std::result::Result<(), String> {
+    if s.active().is_some_and(|d| s.in_group(d.uid)) { Err("finish or stop the running action first".into()) } else { Ok(()) }
+}
 pub fn has_clipboard(s: &Session) -> std::result::Result<(), String> {
     has_doc(s)?;
     if s.clipboard.is_some() { Ok(()) } else { Err("the clipboard is empty".into()) }

crates/engine/src/cmd/sheet.rs

@@ -554,11 +554,15 @@ fn insert_copied_cells(s: &mut Session, p: &Json) -> Result<Json> {
     }
     let revision = d.revision;
     let before = (d.wb.clone(), d.selection.clone());
+    // Inside an undo group the nested steps add no undo entries of their own.
+    let nested_entries = !s.in_group(d.uid);
     let result = insert_copied_steps(s, &clip, sheet, at, shift, cut);
     let d = s.doc_mut()?;
     // The nested steps collapse into this command's single undo step.
-    for _ in 0..d.revision.saturating_sub(revision) {
-        d.undo.pop();
+    if nested_entries {
+        for _ in 0..d.revision.saturating_sub(revision) {
+            d.undo.pop();
+        }
     }
     match result {
         Ok((final_range, out)) => {