fix: set_focus/blur_focus args stripped by get_handler_args for stateless handlers (#6314)

get_handler_args() used `len(args) > 1` assuming all handlers have a `self`
parameter from a State class. Server-side events like _set_focus have no state,
so their single `ref` parameter was incorrectly stripped, producing an empty
payload and a frontend TypeError: Cannot read properties of undefined (reading ref).

Author: FarhanAliRaza

packages/reflex-base/src/reflex_base/event/__init__.py

@@ -2096,8 +2096,9 @@ def get_handler_args(
         The handler args.
     """
     args = event_spec.handler._parameters
+    n_self_args = 1 if event_spec.handler.state is not None else 0
 
-    return event_spec.args if len(args) > 1 else ()
+    return event_spec.args if len(args) > n_self_args else ()
 
 
 def fix_events(

tests/integration/test_server_side_event.py

@@ -49,6 +49,7 @@ def index():
             rx.input(default_value="a", id="a"),
             rx.input(default_value="b", id="b"),
             rx.input(default_value="c", id="c"),
+            rx.el.input(name="name", id="focus_target"),
             rx.button(
                 "Clear Immediate",
                 id="clear_immediate",
@@ -78,6 +79,11 @@ def index():
                 id="clear_return_c",
                 on_click=SSState.set_value_return_c,
             ),
+            rx.el.button(
+                "Focus input",
+                id="focus_input",
+                on_click=rx.set_focus("focus_target"),
+            ),
         )
 
 
@@ -183,3 +189,24 @@ def test_set_value_return_c(driver):
     assert input_a.get_attribute("value") == "a"
     assert input_b.get_attribute("value") == "b"
     assert input_c.get_attribute("value") == ""
+
+
+def test_set_focus(driver):
+    """Call set_focus and verify the target input becomes active.
+
+    Args:
+        driver: selenium WebDriver open to the app
+    """
+    input_target = driver.find_element(By.ID, "focus_target")
+    btn = driver.find_element(By.ID, "focus_input")
+
+    assert input_target
+    assert btn
+
+    btn.click()
+
+    assert AppHarness.poll_for_or_raise_timeout(
+        lambda: (
+            driver.execute_script("return document.activeElement?.id") == "focus_target"
+        )
+    )

tests/units/test_event.py

@@ -280,6 +280,33 @@ def test_focus(func: str, qualname: str):
     )
 
 
+@pytest.mark.parametrize(
+    ("func", "qualname"), [("set_focus", "_set_focus"), ("blur_focus", "_blur_focus")]
+)
+def test_focus_event_chain_preserves_args(func: str, qualname: str):
+    """Test that set_focus/blur_focus ref arg survives EventChain.create.
+
+    Args:
+        func: The event function name.
+        qualname: The sig qual name passed to JS.
+    """
+    spec = getattr(event, func)("input1")
+    chain = EventChain.create(value=spec, args_spec=lambda: ())
+    assert isinstance(chain, EventChain)
+    assert len(chain.events) == 1
+    chain_spec = chain.events[0]
+    assert isinstance(chain_spec, EventSpec)
+    # The ref arg must survive the EventChain pipeline.
+    assert len(chain_spec.args) == 1
+    assert chain_spec.args[0][0].equals(Var(_js_expr="ref"))
+    assert chain_spec.args[0][1].equals(LiteralVar.create("ref_input1"))
+    # Verify the serialized output includes the ref in the payload.
+    assert (
+        format.format_event(chain_spec)
+        == f'ReflexEvent("{qualname}", {{ref:"ref_input1"}})'
+    )
+
+
 def test_set_value():
     """Test the event window alert function."""
     spec = event.set_value("input1", "")