Fix tree expand in live mode: use focused item and run IO in background

_on_tree_open used tree.selection() which may differ from the opened
node; switched to tree.focus(). Network calls in _live_tree_expand now
run in a daemon thread so the UI stays responsive during lazy loading.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -1352,13 +1352,13 @@ def attach_tree_scrollbars(container, tree):
     def _on_tree_open(event):
         tree.after_idle(fit_tree_column, tree)
         if app_state.live_mode:
-            sel = tree.selection()
-            if sel:
-                elem = app_state.tree_element_map.get(sel[0])
+            opened = tree.focus()
+            if opened:
+                elem = app_state.tree_element_map.get(opened)
                 if elem is not None:
                     cid = elem.get("id")
                     if cid is not None:
-                        _live_tree_expand(int(cid))
+                        _live_tree_expand(int(cid), opened)
 
     tree.bind("<<TreeviewOpen>>", _on_tree_open, add="+")
     tree.bind("<<TreeviewClose>>", lambda event: tree.after_idle(fit_tree_column, tree), add="+")
@@ -2041,20 +2041,42 @@ def _start_cov_polling():
     _cov_poll_id = app_state.window.after(100, poll)
 
 
-def _live_tree_expand(comp_id):
+def _live_tree_expand(comp_id, tree_item_id):
     """Called when user expands a tree node in live mode to lazy-load children."""
     ds = app_state.device_session
     if ds is None or not ds.is_connected:
         return
 
-    children = ds.load_children(comp_id)
-    if not children:
-        return
+    app_state.set_status("Loading children...")
 
-    sax_root = ds.build_dom_tree(ds.sox_client.cache_get(0))
-    ds.build_links_elements(sax_root)
-    app_state.xml_root_element = sax_root
-    sax_file.refresh_treeview_from_dom(sax_root)
+    def _bg():
+        try:
+            children = ds.load_children(comp_id)
+            if not children:
+                app_state.window.after(0, lambda: app_state.set_status("No children"))
+                return
+            sax_root = ds.build_dom_tree(ds.sox_client.cache_get(0))
+            ds.build_links_elements(sax_root)
+
+            def _ui():
+                app_state.xml_root_element = sax_root
+                sax_file.refresh_treeview_from_dom(sax_root)
+                # Re-open and select the node the user clicked
+                for iid, elem in app_state.tree_element_map.items():
+                    if elem.get("id") == str(comp_id):
+                        tree.item(iid, open=True)
+                        tree.selection_set(iid)
+                        tree.see(iid)
+                        break
+                app_state.set_status(f"Loaded {len(children)} children")
+
+            app_state.window.after(0, _ui)
+        except Exception as exc:
+            app_state.window.after(
+                0, lambda: app_state.set_status(f"Load error: {exc}", warn=True)
+            )
+
+    threading.Thread(target=_bg, daemon=True).start()
 
 
 # --- UI Setup ---