Fix nullable stored-history constructor switching

crates/oa-desktop/src/forms/history_fields.rs

@@ -382,6 +382,45 @@ fn branch_has_fields(schema: &'static Value) -> bool {
             .is_none_or(|values| values.len() > 1)
 }
 
+/// Constructor identity is independent of a primitive's edited content. In
+/// particular, absent, an empty UUID/text, a number and a list are different
+/// branches. String enum tags and externally tagged objects retain their variant.
+pub(super) fn constructor_tag(path: &str, value: &Value) -> String {
+    fn enum_value(schema: &'static Value, value: &Value, depth: usize) -> bool {
+        if depth > MAX_DEPTH {
+            return false;
+        }
+        let Ok(schema) = resolve(schema) else {
+            return false;
+        };
+        if let Some(options) = alternatives(schema) {
+            return options
+                .iter()
+                .any(|option| enum_value(option, value, depth + 1));
+        }
+        schema
+            .get("enum")
+            .and_then(Value::as_array)
+            .is_some_and(|values| values.contains(value))
+    }
+    match value {
+        Value::Null => "null".into(),
+        Value::Bool(_) => "boolean".into(),
+        Value::Number(_) => "number".into(),
+        Value::Array(_) => "array".into(),
+        Value::String(word)
+            if schema_at(path).is_some_and(|schema| enum_value(schema, value, 0)) =>
+        {
+            format!("enum:{word}")
+        }
+        Value::String(_) => "string".into(),
+        Value::Object(fields) => format!(
+            "object:{}",
+            fields.keys().cloned().collect::<Vec<_>>().join("/")
+        ),
+    }
+}
+
 fn paging<'a>(draft: &'a Draft, path: &str, count: usize) -> (usize, View<'a, Message>) {
     let last = count.saturating_sub(1) / PAGE_SIZE;
     let page = draft