[core] Optionally delay useTransitionStatus's entering status with a param

docs/src/app/experiments/collapsible-transitions.tsx

@@ -0,0 +1,77 @@
+'use client';
+import * as React from 'react';
+import { Collapsible } from '@base-ui-components/react/collapsible';
+import classes from './collapsible.module.css';
+
+function classNames(...c: Array<string | undefined | null | false>) {
+  return c.filter(Boolean).join(' ');
+}
+
+export default function CollapsibleTransitions() {
+  return (
+    <div className={classes.grid}>
+      <div>
+        <Collapsible.Root defaultOpen={false}>
+          <Collapsible.Trigger className={classes.trigger}>
+            <ExpandMoreIcon className={classes.icon} />
+            Trigger 1A
+          </Collapsible.Trigger>
+          <Collapsible.Panel
+            className={classNames(classes.panel, classes.transition)}
+          >
+            <p>This is the collapsed content</p>
+            <p>
+              You can find the Base UI repository{' '}
+              <a
+                href="https://github.com/mui/base-ui"
+                target="_blank"
+                rel="noreferrer noopener"
+              >
+                here
+              </a>
+            </p>
+          </Collapsible.Panel>
+        </Collapsible.Root>
+      </div>
+
+      <div>
+        <Collapsible.Root>
+          <Collapsible.Trigger className={classes.trigger}>
+            <ExpandMoreIcon className={classes.icon} />
+            Trigger 1B
+          </Collapsible.Trigger>
+          <Collapsible.Panel
+            className={classNames(classes.panel, classes.transition)}
+          >
+            <p>This is the collapsed content</p>
+            <p>
+              You can find the Base UI repository{' '}
+              <a
+                href="https://github.com/mui/base-ui"
+                target="_blank"
+                rel="noreferrer noopener"
+              >
+                here
+              </a>
+            </p>
+          </Collapsible.Panel>
+        </Collapsible.Root>
+      </div>
+    </div>
+  );
+}
+
+function ExpandMoreIcon(props: React.SVGProps<SVGSVGElement>) {
+  return (
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      {...props}
+      width="24"
+      height="24"
+      viewBox="0 0 24 24"
+      fill="none"
+    >
+      <path d="M16.59 8.59 12 13.17 7.41 8.59 6 10l6 6 6-6z" fill="currentColor" />
+    </svg>
+  );
+}

docs/src/app/experiments/collapsible.module.css

@@ -33,7 +33,7 @@
   transition: transform var(--duration) ease-in;
 }
 
-.trigger[data-open] .icon {
+.trigger[data-panel-open] .icon {
   transform: rotate(0);
   transition: transform var(--duration) ease-out;
 }
@@ -96,3 +96,15 @@
     height: 0;
   }
 }
+
+.grid {
+  --width: 320px;
+  --duration: 2000ms;
+
+  font-family: system-ui, sans-serif;
+  line-height: 1.4;
+
+  display: grid;
+  grid: var(--width) var(--width) / var(--width) var(--width);
+  grid-gap: 4rem;
+}

packages/react/src/collapsible/root/useCollapsibleRoot.ts

@@ -23,7 +23,7 @@ export function useCollapsibleRoot(
     state: 'open',
   });
 
-  const { mounted, setMounted, transitionStatus } = useTransitionStatus(open, animated);
+  const { mounted, setMounted, transitionStatus } = useTransitionStatus(open, animated, true);
 
   const [panelId, setPanelId] = React.useState<string | undefined>(useId());
 

packages/react/src/utils/useTransitionStatus.ts

@@ -8,9 +8,12 @@ export type TransitionStatus = 'entering' | 'exiting' | undefined;
  * Provides a status string for CSS animations.
  * @param open - a boolean that determines if the element is open.
  * @param enabled - a boolean that determines if the logic is enabled.
+ * @param delayEnteringStatus - a boolean that set the `entering` status one
+ *     tick later. Example use-case: collapsible needs an extra frame in order
+ *     to measure the panel contents.
  * @ignore - internal hook.
  */
-export function useTransitionStatus(open: boolean, enabled = true) {
+export function useTransitionStatus(open: boolean, enabled = true, delayEnteringStatus = false) {
   const [transitionStatus, setTransitionStatus] = React.useState<TransitionStatus>();
   const [mounted, setMounted] = React.useState(open);
 
@@ -19,7 +22,7 @@ export function useTransitionStatus(open: boolean, enabled = true) {
   if (enabled) {
     if (open && !mounted) {
       setMounted(true);
-      if (transitionStatus !== 'entering') {
+      if (transitionStatus !== 'entering' && !delayEnteringStatus) {
         setTransitionStatus('entering');
       }
     }
@@ -38,14 +41,18 @@ export function useTransitionStatus(open: boolean, enabled = true) {
       return undefined;
     }
 
+    if (delayEnteringStatus) {
+      setTransitionStatus('entering');
+    }
+
     const frame = requestAnimationFrame(() => {
       setTransitionStatus(undefined);
     });
 
     return () => {
       cancelAnimationFrame(frame);
     };
-  }, [enabled, open]);
+  }, [enabled, open, delayEnteringStatus]);
 
   return React.useMemo(
     () => ({