chore(Accordion): fix defaultOpen flicker on first render (#2214)

resolves #2178

.changeset/chilled-icons-chew.md

@@ -0,0 +1,5 @@
+---
+"@digdir/designsystemet-react": patch
+---
+
+Accordion: Fix `defaultOpen` flicker on first render

packages/react/src/utilities/AnimateHeight/AnimateHeight.test.tsx

@@ -35,7 +35,7 @@ describe('AnimateHeight', () => {
   it('Appends given style to root element', () => {
     const style = { color: 'rgb(255, 0, 0)' };
     const { container } = render({ style });
-    expect(container.firstChild).toHaveStyle({ height: 0 });
+    expect(container.firstChild).toHaveStyle({ height: undefined });
     expect(container.firstChild).toHaveStyle(style);
   });
 

packages/react/src/utilities/AnimateHeight/AnimateHeight.tsx

@@ -22,7 +22,10 @@ export const AnimateHeight = ({
   style,
   ...rest
 }: AnimateHeightProps) => {
-  const [height, setHeight] = useState<number>(0);
+  /* We don't know the initial height we want to start with.
+  It depends on if it should start open or not, therefore we set height to `undefined`,
+  so we don't get any layoutshift on first render */
+  const [height, setHeight] = useState<number | undefined>(undefined);
   const prevOpen = usePrevious(open);
   const openOrClosed: InternalState = open ? 'open' : 'closed';
   const [state, setState] = useState<InternalState>(openOrClosed);