Handle out of order blocks when parsing element tree (#7923)

lib/streamlit/testing/v1/element_tree.py

@@ -1915,6 +1915,13 @@ def parse_tree_from_messages(messages: list[ForwardMsg]) -> ElementTree:
                 children[idx] = child
             assert isinstance(child, Block)
             current_node = child
+
+        # Handle a block when we already have a placeholder for that location
+        if isinstance(new_node, Block):
+            placeholder_block = current_node.children.get(delta_path[-1])
+            if placeholder_block is not None:
+                new_node.children = placeholder_block.children
+
         current_node.children[delta_path[-1]] = new_node
 
     return root

lib/tests/streamlit/testing/app_test_test.py

@@ -181,3 +181,27 @@ def script():
     assert len(at.text) == 4
     # querying elements via a block only returns the elements in that block
     assert len(at.get("expandable")[0].text) == 2
+
+
+def test_out_of_order_blocks() -> None:
+    # Regression test for #7711
+    def script():
+        import streamlit as st
+
+        container = st.container()
+        with container:
+            st.markdown("BarFoo")
+
+            def button_one_clicked(cont):
+                cont.info("Hi!")
+                cont.markdown("FooBar")
+
+            st.button("one", on_click=button_one_clicked, args=[container])
+
+    at = AppTest.from_function(script).run()
+
+    at.button[0].click().run()
+
+    assert at.markdown.len == 2
+    assert at.info[0].value == "Hi!"
+    assert at.markdown.values == ["FooBar", "BarFoo"]