Fix multi set_props in background callbacks.

Author: T4rk1n

dash/long_callback/_proxy_set_props.py

@@ -7,6 +7,12 @@ class ProxySetProps(dict):
     def __init__(self, on_change):
         super().__init__()
         self.on_change = on_change
+        self._data = {}
 
     def __setitem__(self, key, value):
         self.on_change(key, value)
+        self._data.setdefault(key, {})
+        self._data[key] = {**self._data[key], **value}
+
+    def get(self, key):
+        return self._data.get(key)

tests/integration/long_callback/app_arbitrary.py

@@ -25,9 +25,11 @@
     Input("start", "n_clicks"),
     prevent_initial_call=True,
     background=True,
+    interval=500,
 )
 def on_click(_):
     set_props("secondary", {"children": "first"})
+    set_props("secondary", {"style": {"background": "red"}})
     time.sleep(2)
     set_props("secondary", {"children": "second"})
     return "completed"

tests/integration/long_callback/test_basic_long_callback017.py

@@ -8,6 +8,9 @@ def test_lcbc017_long_callback_set_props(dash_duo, manager):
         dash_duo.find_element("#start").click()
 
         dash_duo.wait_for_text_to_equal("#secondary", "first")
+        dash_duo.wait_for_style_to_equal(
+            "#secondary", "background-color", "rgba(255, 0, 0, 1)"
+        )
         dash_duo.wait_for_text_to_equal("#output", "initial")
         dash_duo.wait_for_text_to_equal("#secondary", "second")
         dash_duo.wait_for_text_to_equal("#output", "completed")