Fix multi set_props calls

Author: T4rk1n

dash/_callback_context.py

@@ -252,7 +252,11 @@ def timing_information(self):
     def set_props(self, component_id: typing.Union[str, dict], props: dict):
         ctx_value = _get_context_value()
         _id = stringify_id(component_id)
-        ctx_value.updated_props[_id] = props
+        updates = ctx_value.updated_props.get(_id)
+        if updates is not None:
+            ctx_value.updated_props[_id] = {**updates, **props}
+        else:
+            ctx_value.updated_props[_id] = props
 
 
 callback_context = CallbackContext()

tests/integration/callbacks/test_arbitrary_callbacks.py

@@ -165,3 +165,26 @@ def on_click(clicked):
         ".dash-fe-error__title",
         "Callback error with no output from input start.n_clicks",
     )
+
+
+def test_arb006_multi_set_props(dash_duo):
+    app = Dash()
+
+    app.layout = [
+        html.Button("start", id="start"),
+        html.Div("initial", id="output"),
+    ]
+
+    @app.callback(
+        Input("start", "n_clicks"),
+    )
+    def on_click(_):
+        set_props("output", {"children": "changed"})
+        set_props("output", {"style": {"background": "rgb(255,0,0)"}})
+
+    dash_duo.start_server(app)
+    dash_duo.wait_for_element("#start").click()
+    dash_duo.wait_for_text_to_equal("#output", "changed")
+    dash_duo.wait_for_style_to_equal(
+        "#output", "background-color", "rgba(255, 0, 0, 1)"
+    )