Merge branch 'dev' into fix/ws-connection-management

Author: T4rk1n

CHANGELOG.md

@@ -23,6 +23,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - [#3768](https://github.com/plotly/dash/pull/3768) Improved `Dropdown` search performance for large options lists
 - [#3759](https://github.com/plotly/dash/pull/3759) Fix the issue where `Patch` objects cannot be updated via `set_props()` in `websocket` callback. Fix [#3742](https://github.com/plotly/dash/issues/3742)
 - [#3789](https://github.com/plotly/dash/pull/3789) Fixed extra wrapper in `DatePickerRange` and `DatePickerSingle` causing styling and layout issues.
+- [#3799](https://github.com/plotly/dash/pull/3799) Fixed dropdown crash when filtering large datasets with component-based labels by using stable item keys for virtualized rows.
 
 ## [4.2.0rc3] - 2026-05-12
 

components/dash-core-components/src/utils/optionRendering.tsx

@@ -400,6 +400,9 @@ export const OptionsList = forwardRef<OptionsListHandle, OptionsListProps>(
                     className="dash-options-list-virtualized"
                     onItemsRendered={handleItemsRendered}
                     itemData={itemData}
+                    itemKey={(index, data) =>
+                        String(data.options[index]?.value ?? index)
+                    }
                 >
                     {Row}
                 </VariableSizeList>

components/dash-core-components/tests/integration/dropdown/test_search_value.py

@@ -107,3 +107,41 @@ def send_key(key):
     last_option.click()
 
     dash_duo.wait_for_text_to_equal("#output", "value=opt_100")
+
+
+def test_ddsv003_dropdown_virtualized_component_label_filtering(dash_duo):
+    app = Dash(__name__)
+
+    options = [
+        {
+            "label": html.Div(["Item ", html.Span(f"#{i}")]),
+            "value": f"item_{i}",
+            "search": f"item_{i}",
+        }
+        for i in range(1, 2000)
+    ]
+
+    app.layout = html.Div(
+        [
+            dcc.Dropdown(
+                id="dd",
+                options=options,
+                value="item_1",
+                searchable=True,
+                clearable=True,
+            )
+        ]
+    )
+
+    dash_duo.start_server(app)
+
+    dropdown = dash_duo.find_element("#dd")
+    dropdown.click()
+
+    search = dash_duo.find_element(".dash-dropdown-search")
+
+    # trigger filtering path that used to crash virtualized list
+    search.send_keys("199")
+    sleep(0.5)
+
+    assert dash_duo.get_logs() == []

dash/backends/_quart.py

@@ -122,7 +122,6 @@ def create_app(
     def register_assets_blueprint(
         self, blueprint_name: str, assets_url_path: str, assets_folder: str  # type: ignore[name-defined]
     ):
-
         bp = Blueprint(
             blueprint_name,
             __name__,

dash/dash.py

@@ -135,6 +135,8 @@
 _ID_STORE = "_pages_store"
 _ID_DUMMY = "_pages_dummy"
 
+_UNINITIALIZED = object()  # Sentinel for tracking init_app state
+
 DASH_VERSION_URL = "https://dash-version.plotly.com:8080/current_version"
 
 # Handles the case in a newly cloned environment where the components are not yet generated.
@@ -731,6 +733,17 @@ def init_app(self, app: Optional[Any] = None, **kwargs) -> None:
         )
         if app is not None:
             self.server = app
+            # Also update the backend's server reference so routes are registered
+            # on the correct server (important when using server=False pattern)
+            self.backend.server = app
+
+        # Skip registration if already initialized on this server
+        # This prevents double registration when init_app() is called multiple times
+        # (e.g., with flask run pattern where __init__ calls init_app, then user does too)
+        if getattr(self, "_initialized_server", _UNINITIALIZED) is self.server:
+            return
+        self._initialized_server = self.server
+
         bp_prefix = config.routes_pathname_prefix.replace("/", "_").replace(".", "_")
         assets_blueprint_name = f"{bp_prefix}dash_assets"
         self.backend.register_assets_blueprint(

package-lock.json

@@ -524,3 +524,43 @@ def test_csrf_config_read_only():
         app.config.csrf_token_name = "something_else"
     with pytest.raises(AttributeError):
         app.config.csrf_header_name = "something_else"
+
+
+def test_init_app_with_flask_run_pattern():
+    """Test that init_app works correctly with the flask run pattern.
+
+    This tests the fix for https://github.com/plotly/dash/issues/3787
+    where using flask run would cause a ValueError about duplicate blueprint
+    registration because init_app was called twice (once automatically in
+    __init__ with server=True, and once by the user in their create_app factory).
+    """
+    # Simulate the flask run pattern where:
+    # 1. Dash app is created with server=True (default)
+    # 2. User's create_app factory calls init_app with their Flask server
+    external_server = Flask("external_test")
+    app = Dash(__name__)
+
+    # This should NOT raise "ValueError: The name '_dash_assets' is already registered"
+    app.init_app(external_server)
+
+    # Verify the backend now uses the external server
+    assert app.server is external_server
+    assert app.backend.server is external_server
+
+
+def test_init_app_server_false_pattern():
+    """Test that init_app works correctly when server=False is used.
+
+    This tests the fix for https://github.com/plotly/dash/issues/3787
+    where using server=False and then calling init_app would result in
+    404 errors because the backend's server reference was not updated.
+    """
+    external_server = Flask("external_test_false")
+    app = Dash(__name__, server=False)
+
+    # Call init_app with the external server
+    app.init_app(external_server)
+
+    # Verify both the app and backend use the external server
+    assert app.server is external_server
+    assert app.backend.server is external_server