diff --git a/packages/react/src/select/item/SelectItem.tsx b/packages/react/src/select/item/SelectItem.tsx
index 29f8fe8afcf..355bca0e8fd 100644
--- a/packages/react/src/select/item/SelectItem.tsx
+++ b/packages/react/src/select/item/SelectItem.tsx
@@ -42,9 +42,11 @@ export const SelectItem = React.memo(
     } = componentProps;
 
     const textRef = React.useRef<HTMLElement | null>(null);
+    const metadata = React.useMemo(() => ({ value: itemValue }), [itemValue]);
     const listItem = useCompositeListItem({
       guess: true,
       label,
+      metadata,
       textRef,
     });
 
@@ -55,7 +57,6 @@ export const SelectItem = React.memo(
       setValue,
       selectionRef,
       typingRef,
-      valuesRef,
       multiple,
       selectedItemTextRef,
       disabled: selectDisabled,
@@ -72,15 +73,6 @@ export const SelectItem = React.memo(
 
     const itemRef = React.useRef<HTMLDivElement | null>(null);
 
-    useIsoLayoutEffect(() => {
-      const values = valuesRef.current;
-      values[index] = itemValue;
-
-      return () => {
-        delete values[index];
-      };
-    }, [index, itemValue, valuesRef]);
-
     useIsoLayoutEffect(() => {
       const selectedValue = store.state.value;
 
diff --git a/packages/react/src/select/positioner/SelectPositioner.tsx b/packages/react/src/select/positioner/SelectPositioner.tsx
index f6f46e725a8..2f1324ed69e 100644
--- a/packages/react/src/select/positioner/SelectPositioner.tsx
+++ b/packages/react/src/select/positioner/SelectPositioner.tsx
@@ -5,7 +5,10 @@ import { useIsoLayoutEffect } from '@base-ui/utils/useIsoLayoutEffect';
 import { useStableCallback } from '@base-ui/utils/useStableCallback';
 import { useStore } from '@base-ui/utils/store';
 import { useSelectRootContext } from '../root/SelectRootContext';
-import { CompositeList } from '../../internals/composite/list/CompositeList';
+import {
+  CompositeList,
+  type CompositeMetadata,
+} from '../../internals/composite/list/CompositeList';
 import type { BaseUIComponentProps } from '../../internals/types';
 import {
   useAnchorPositioning,
@@ -151,67 +154,66 @@ export const SelectPositioner = React.forwardRef(function SelectPositioner(
     inert: !open,
   });
 
-  const prevMapSizeRef = React.useRef(0);
-
-  const onMapChange = useStableCallback(
-    (map: Map<Element, { index?: number | null | undefined } | null>) => {
-      if (valuesRef.current.length === 0) {
-        return;
-      }
+  const onMapChange = useStableCallback((map: Map<Element, CompositeMetadata<{ value: any }>>) => {
+    const previousValues = valuesRef.current;
+    const nextValues = Array.from(map.values(), (item) => item.value);
+    valuesRef.current = nextValues;
 
-      const prevSize = prevMapSizeRef.current;
-      prevMapSizeRef.current = map.size;
-
-      if (map.size === prevSize) {
-        return;
-      }
-
-      const eventDetails = createChangeEventDetails(REASONS.none);
-
-      if (prevSize !== 0 && !store.state.multiple && value !== null) {
-        const selectedValueIndex = findItemIndex(valuesRef.current, value, isItemEqualToValue);
-        if (selectedValueIndex === -1) {
-          const initialSelectedValue = initialValueRef.current;
-          const hasInitial =
-            initialSelectedValue != null &&
-            findItemIndex(valuesRef.current, initialSelectedValue, isItemEqualToValue) !== -1;
-          const nextValue = hasInitial ? initialSelectedValue : null;
-          setValue(nextValue, eventDetails);
+    if (nextValues.length === previousValues.length) {
+      return;
+    }
 
-          if (nextValue === null) {
-            store.set('selectedIndex', null);
-            selectedItemTextRef.current = null;
-          }
+    const eventDetails = createChangeEventDetails(REASONS.none);
+
+    if (previousValues.length !== 0 && !store.state.multiple && value !== null) {
+      const selectedValueIndex = findItemIndex(nextValues, value, isItemEqualToValue);
+      if (selectedValueIndex === -1) {
+        const initialSelectedValue = initialValueRef.current;
+        const hasInitial =
+          initialSelectedValue != null &&
+          findItemIndex(nextValues, initialSelectedValue, isItemEqualToValue) !== -1;
+        const nextValue = hasInitial ? initialSelectedValue : null;
+        setValue(nextValue, eventDetails);
+
+        if (nextValue === null) {
+          store.set('selectedIndex', null);
+          selectedItemTextRef.current = null;
         }
       }
+    }
 
-      if (prevSize !== 0 && store.state.multiple && Array.isArray(value)) {
-        const nextValue = value.filter(
-          (selectedItemValue) =>
-            findItemIndex(valuesRef.current, selectedItemValue, isItemEqualToValue) !== -1,
-        );
-        if (nextValue.length !== value.length) {
-          setValue(nextValue, eventDetails);
-
-          if (nextValue.length === 0) {
-            store.set('selectedIndex', null);
-            selectedItemTextRef.current = null;
-          }
+    if (previousValues.length !== 0 && store.state.multiple && Array.isArray(value)) {
+      const nextValue = value.filter(
+        (selectedItemValue) =>
+          findItemIndex(nextValues, selectedItemValue, isItemEqualToValue) !== -1,
+      );
+      if (nextValue.length !== value.length) {
+        setValue(nextValue, eventDetails);
+
+        if (nextValue.length === 0) {
+          store.set('selectedIndex', null);
+          selectedItemTextRef.current = null;
         }
       }
+    }
 
-      if (open && alignItemWithTriggerActive) {
-        store.update({
-          scrollUpArrowVisible: false,
-          scrollDownArrowVisible: false,
-        });
+    if (open && alignItemWithTriggerActive) {
+      store.update({
+        scrollUpArrowVisible: false,
+        scrollDownArrowVisible: false,
+      });
 
-        const stylesToClear: React.CSSProperties = { height: '' };
-        clearStyles(positionerElement, stylesToClear);
-        clearStyles(popupRef.current, stylesToClear);
-      }
-    },
-  );
+      const stylesToClear: React.CSSProperties = { height: '' };
+      clearStyles(positionerElement, stylesToClear);
+      clearStyles(popupRef.current, stylesToClear);
+    }
+  });
+
+  useIsoLayoutEffect(() => {
+    return () => {
+      valuesRef.current = [];
+    };
+  }, [valuesRef]);
 
   const contextValue: SelectPositionerContext = React.useMemo(
     () => ({
@@ -226,7 +228,11 @@ export const SelectPositioner = React.forwardRef(function SelectPositioner(
   );
 
   return (
-    <CompositeList elementsRef={listRef} labelsRef={labelsRef} onMapChange={onMapChange}>
+    <CompositeList<{ value: any }>
+      elementsRef={listRef}
+      labelsRef={labelsRef}
+      onMapChange={onMapChange}
+    >
       <SelectPositionerContext.Provider value={contextValue}>
         {mounted && modal && <InternalBackdrop inert={inertValue(!open)} cutout={triggerElement} />}
         {element}
diff --git a/packages/react/src/tabs/panel/TabsPanel.test.tsx b/packages/react/src/tabs/panel/TabsPanel.test.tsx
index 87213e7aa50..3f5671c3623 100644
--- a/packages/react/src/tabs/panel/TabsPanel.test.tsx
+++ b/packages/react/src/tabs/panel/TabsPanel.test.tsx
@@ -25,6 +25,36 @@ describe('<Tabs.Panel />', () => {
   });
 
   describe('panels sharing a value', () => {
+    it('gives ownership to a panel that starts sharing the value later', async () => {
+      function App() {
+        const [firstValue, setFirstValue] = React.useState('a');
+
+        return (
+          <React.Fragment>
+            <button type="button" onClick={() => setFirstValue('b')}>
+              share value
+            </button>
+            <Tabs.Root value="c">
+              <Tabs.List>
+                <Tabs.Tab value="b">B</Tabs.Tab>
+              </Tabs.List>
+              <Tabs.Panel value={firstValue} keepMounted data-testid="first" />
+              <Tabs.Panel value="b" keepMounted data-testid="second" />
+            </Tabs.Root>
+          </React.Fragment>
+        );
+      }
+
+      const { user } = await render(<App />);
+      const tab = screen.getByRole('tab');
+
+      expect(tab).toHaveAttribute('aria-controls', screen.getByTestId('second').id);
+
+      await user.click(screen.getByRole('button', { name: 'share value' }));
+
+      expect(tab).toHaveAttribute('aria-controls', screen.getByTestId('first').id);
+    });
+
     it('keeps the surviving registration when a shadowed panel unmounts', async () => {
       function App() {
         const [shadowedMounted, setShadowedMounted] = React.useState(true);