Engine: a JSON workbook with no sheets keeps its macros

The JSON reader replaces a workbook with an empty sheet list by a new
one before macros::load reads the file's macro entries, so they were
lost with no warning. The entries are carried across the reset. The
reset itself, which also starts the names and styles afresh, is
unchanged.

crates/engine/src/io.rs

@@ -166,7 +166,10 @@ fn read_plain(name: &str, bytes: &[u8], csv_as_text: bool) -> Result<(Workbook,
             let mut wb: Workbook = serde_json::from_slice(bytes).map_err(|e| EngineError::Other(format!("not a GridCraft JSON workbook: {e}")))?;
             wb.styles.rebuild_index();
             if wb.sheets.is_empty() {
+                // The file's macros are read after this (`macros::load`), so they come along.
+                let macros = std::mem::take(&mut wb.macros_read);
                 wb = Workbook::new();
+                wb.macros_read = macros;
             }
             // The file's band placement and cell count are derived state: rebuild them from the
             // cells themselves, dropping what is outside the sheet grid (#1008).

crates/engine/src/macros/tests.rs

@@ -514,3 +514,16 @@ fn naming_clashing_macros_takes_linear_work() {
     taken.insert("a (2)");
     assert_eq!(taken.unique("a"), "a (4)");
 }
+
+#[test]
+fn a_json_workbook_with_no_sheets_keeps_its_macros() {
+    // A JSON file with an empty sheet list opens as a new workbook; its macros come along.
+    let mut wb = Workbook::new();
+    wb.macros.push(sample_macro("A"));
+    let mut v: serde_json::Value = serde_json::from_slice(&crate::io::save_bytes(&wb, "book.json").unwrap()).unwrap();
+    v["sheets"] = json!([]);
+    let (back, warnings) = crate::io::open_bytes("book.json", v.to_string().as_bytes()).unwrap();
+    assert!(warnings.is_empty(), "{warnings:?}");
+    assert_eq!(back.sheets.len(), 1);
+    assert_eq!(back.macros, wb.macros);
+}