Review nested message kind groups with exact timer scope

crates/oa-desktop/src/forms.rs

@@ -46,6 +46,7 @@ pub struct Draft {
     pub inputs: BTreeMap<String, Input>,
     pub exact_strings: bool,
     context: Option<Value>,
+    message_timer_draft: Option<Input>,
     port_contract_drafts: BTreeMap<String, Value>,
     pub value_browsers: BTreeMap<String, ValueBrowser>,
 }
@@ -85,6 +86,7 @@ impl Draft {
             inputs: BTreeMap::new(),
             exact_strings: false,
             context: None,
+            message_timer_draft: None,
             port_contract_drafts: BTreeMap::new(),
             value_browsers: BTreeMap::new(),
         };
@@ -248,6 +250,29 @@ impl Draft {
         self.rebuild_retaining(|old| (!within(old, path)).then(|| old.to_owned()));
         Ok(())
     }
+    /// Message-kind selection preserves the timer's exact input/editor state
+    /// while its payload is temporarily hidden by another kind.
+    pub fn message_kind_variant(&mut self, path: &str, value: Value) -> Result<(), String> {
+        serde_json::from_value::<MessageKind>(value.clone()).map_err(|e| e.to_string())?;
+        let current = self
+            .value
+            .pointer(path)
+            .ok_or("Message kind no longer exists.")?;
+        if same_variant(current, &value) {
+            return Ok(());
+        }
+        let timer_path = format!("{path}/Timer/expression");
+        if let Some(input) = self.inputs.remove(&timer_path) {
+            self.message_timer_draft = Some(input);
+        }
+        self.shape_scoped(path, value)?;
+        if self.inputs.contains_key(&timer_path)
+            && let Some(input) = self.message_timer_draft.take()
+        {
+            self.inputs.insert(timer_path, input);
+        }
+        Ok(())
+    }
     fn rebuild_retaining(&mut self, new_path: impl Fn(&str) -> Option<String>) {
         let previous = std::mem::take(&mut self.inputs);
         self.rebuild();