dcc redesign: Do not resort dropdown when multi=False

components/dash-core-components/src/components/css/dropdown.css

@@ -5,6 +5,7 @@
     padding: 0;
     background: inherit;
     border: none;
+    outline: none;
     width: 100%;
     cursor: pointer;
     font-size: inherit;
@@ -98,6 +99,8 @@
 }
 
 .dash-dropdown-search-container {
+    position: sticky;
+    top: calc(var(--Dash-Spacing) * 2);
     margin: calc(var(--Dash-Spacing) * 2);
     padding: var(--Dash-Spacing);
     border-radius: 4px;

components/dash-core-components/src/fragments/Dropdown.tsx

@@ -44,6 +44,9 @@ const Dropdown = (props: DropdownProps) => {
     const [displayOptions, setDisplayOptions] = useState<DetailedOption[]>([]);
     const persistentOptions = useRef<DropdownProps['options']>([]);
     const dropdownContainerRef = useRef<HTMLButtonElement>(null);
+    const dropdownContentRef = useRef<HTMLDivElement>(
+        document.createElement('div')
+    );
 
     const ctx = window.dash_component_api.useDashContext();
     const loading = ctx.useLoading();
@@ -208,23 +211,46 @@ const Dropdown = (props: DropdownProps) => {
     // Update display options when filtered options or selection changes
     useEffect(() => {
         if (isOpen) {
-            // Sort filtered options: selected first, then unselected
-            const sortedOptions = [...filteredOptions].sort((a, b) => {
-                const aSelected = sanitizedValues.includes(a.value);
-                const bSelected = sanitizedValues.includes(b.value);
+            let sortedOptions = filteredOptions;
+            if (multi) {
+                // Sort filtered options: selected first, then unselected
+                sortedOptions = [...filteredOptions].sort((a, b) => {
+                    const aSelected = sanitizedValues.includes(a.value);
+                    const bSelected = sanitizedValues.includes(b.value);
 
-                if (aSelected && !bSelected) {
-                    return -1;
-                }
-                if (!aSelected && bSelected) {
-                    return 1;
-                }
-                return 0; // Maintain original order within each group
-            });
+                    if (aSelected && !bSelected) {
+                        return -1;
+                    }
+                    if (!aSelected && bSelected) {
+                        return 1;
+                    }
+                    return 0; // Maintain original order within each group
+                });
+            }
 
             setDisplayOptions(sortedOptions);
         }
-    }, [filteredOptions, isOpen]); // Removed sanitizedValues to prevent re-sorting on selection changes
+    }, [filteredOptions, isOpen]);
+
+    // Focus (and scroll) the first selected item when dropdown opens
+    useEffect(() => {
+        if (!isOpen || multi || search_value) {
+            return;
+        }
+
+        // waiting for the DOM to be ready after the dropdown renders
+        requestAnimationFrame(() => {
+            const selectedValue = sanitizedValues[0];
+
+            const selectedElement = dropdownContentRef.current.querySelector(
+                `.dash-options-list-option-checkbox[value="${selectedValue}"]`
+            );
+
+            if (selectedElement instanceof HTMLElement) {
+                selectedElement?.focus();
+            }
+        });
+    }, [isOpen, multi, displayOptions, sanitizedValues]);
 
     // Handle keyboard navigation in popover
     const handleKeyDown = useCallback((e: React.KeyboardEvent) => {
@@ -300,10 +326,16 @@ const Dropdown = (props: DropdownProps) => {
 
         if (nextIndex > -1) {
             focusableElements[nextIndex].focus();
-            focusableElements[nextIndex].scrollIntoView({
-                behavior: 'auto',
-                block: 'center',
-            });
+            if (nextIndex === 0) {
+                // first element is a sticky search bar, so if we are focusing
+                // on that, also move the scroll to the top
+                dropdownContentRef.current?.scrollTo({top: 0});
+            } else {
+                focusableElements[nextIndex].scrollIntoView({
+                    behavior: 'auto',
+                    block: 'center',
+                });
+            }
         }
     }, []);
 
@@ -312,33 +344,7 @@ const Dropdown = (props: DropdownProps) => {
         (open: boolean) => {
             setIsOpen(open);
 
-            if (open) {
-                // Sort options: selected first, then unselected
-                const selectedOptions: DetailedOption[] = [];
-                const unselectedOptions: DetailedOption[] = [];
-
-                // First, collect selected options in the order they appear in the `value` array
-                sanitizedValues.forEach(value => {
-                    const option = filteredOptions.find(
-                        opt => opt.value === value
-                    );
-                    if (option) {
-                        selectedOptions.push(option);
-                    }
-                });
-
-                // Then, collect unselected options in the order they appear in `options` array
-                filteredOptions.forEach(option => {
-                    if (!sanitizedValues.includes(option.value)) {
-                        unselectedOptions.push(option);
-                    }
-                });
-                const sortedOptions = [
-                    ...selectedOptions,
-                    ...unselectedOptions,
-                ];
-                setDisplayOptions(sortedOptions);
-            } else {
+            if (!open) {
                 setProps({search_value: undefined});
             }
         },
@@ -416,6 +422,7 @@ const Dropdown = (props: DropdownProps) => {
 
             <Popover.Portal>
                 <Popover.Content
+                    ref={dropdownContentRef}
                     className="dash-dropdown-content"
                     align="start"
                     sideOffset={5}

components/dash-core-components/tests/integration/dropdown/test_a11y.py

@@ -126,3 +126,88 @@ def send_keys(key):
     assert dash_duo.find_element(".dash-dropdown-value").text == "1, 91"
 
     assert dash_duo.get_logs() == []
+
+
+def test_a11y004_selection_visibility_single(dash_duo):
+    app = Dash(__name__)
+    app.layout = (
+        Dropdown(
+            id="dropdown",
+            options=[f"Option {i}" for i in range(0, 100)],
+            value="Option 71",
+            multi=False,
+            placeholder="Testing selected item is visible on open",
+        ),
+    )
+
+    dash_duo.start_server(app)
+
+    dash_duo.wait_for_element("#dropdown")
+
+    dash_duo.find_element("#dropdown").click()
+    dash_duo.wait_for_element(".dash-dropdown-options")
+
+    # Assert that the selected option is visible in the dropdown
+    selected_option = dash_duo.find_element(".dash-dropdown-option.selected")
+    assert selected_option.text == "Option 71"
+    assert selected_option.is_displayed()
+
+    assert elements_are_visible(
+        dash_duo, selected_option
+    ), "Selected option should be visible when the dropdown opens"
+
+    assert dash_duo.get_logs() == []
+
+
+def test_a11y005_selection_visibility_multi(dash_duo):
+    app = Dash(__name__)
+    app.layout = (
+        Dropdown(
+            id="dropdown",
+            options=[f"Option {i}" for i in range(0, 100)],
+            value=[
+                "Option 71",
+                "Option 23",
+                "Option 42",
+            ],
+            multi=True,
+            placeholder="Testing selected item is visible on open",
+        ),
+    )
+
+    dash_duo.start_server(app)
+
+    dash_duo.wait_for_element("#dropdown")
+
+    dash_duo.find_element("#dropdown").click()
+    dash_duo.wait_for_element(".dash-dropdown-options")
+
+    # Assert that the selected option is visible in the dropdown
+    selected_options = dash_duo.find_elements(".dash-dropdown-option.selected")
+    assert elements_are_visible(
+        dash_duo, selected_options
+    ), "Selected options should be visible when the dropdown opens"
+
+    assert dash_duo.get_logs() == []
+
+
+def elements_are_visible(dash_duo, elements):
+    # Check if the given elements are within the visible viewport of the dropdown
+    elements = elements if isinstance(elements, list) else [elements]
+    dropdown_content = dash_duo.find_element(".dash-dropdown-content")
+
+    def is_visible(el):
+        return dash_duo.driver.execute_script(
+            """
+            const option = arguments[0];
+            const container = arguments[1];
+            const optionRect = option.getBoundingClientRect();
+            const containerRect = container.getBoundingClientRect();
+            return optionRect.top >= containerRect.top &&
+                optionRect.bottom <= containerRect.bottom;
+            """,
+            el,
+            dropdown_content,
+        )
+
+    return all([is_visible(el) for el in elements])

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

@@ -1,3 +1,4 @@
+from time import sleep
 from dash import Dash
 from dash.dcc import Dropdown
 from dash.html import Div
@@ -39,11 +40,13 @@ def test_ddlo001_translations(dash_duo):
     )
 
     dash_duo.find_element(".dash-dropdown-search").send_keys(1)
+    sleep(0.1)
     assert dash_duo.find_element(".dash-dropdown-clear").accessible_name == "Annuler"
 
     dash_duo.find_element(".dash-dropdown-action-button:first-child").click()
 
     dash_duo.find_element(".dash-dropdown-search").send_keys(9)
+    sleep(0.1)
     assert dash_duo.find_element(".dash-dropdown-option").text == "Aucun d'options"
 
     assert (
@@ -84,13 +87,15 @@ def test_ddlo002_partial_translations(dash_duo):
     assert dash_duo.find_element(".dash-dropdown-search").accessible_name == "Lookup"
 
     dash_duo.find_element(".dash-dropdown-search").send_keys(1)
+    sleep(0.1)
     assert (
         dash_duo.find_element(".dash-dropdown-clear").accessible_name == "Clear search"
     )
 
     dash_duo.find_element(".dash-dropdown-action-button:first-child").click()
 
     dash_duo.find_element(".dash-dropdown-search").send_keys(9)
+    sleep(0.1)
     assert dash_duo.find_element(".dash-dropdown-option").text == "No options found"
 
     assert (