Bug report: Accordion initially expanded in popup renders with `height: 0px`

[kleineroscar]

Accordion groups configured as initially expanded are rendered in the expanded state, but their content is not visible when the Accordion is shown inside a popup.

The header state is correct:

aria-expanded="true"

The content also exists and is fully rendered in the DOM, but the content wrapper receives:

style="height: 0px;"

This leaves the group visually collapsed even though the Accordion considers it expanded.

Steps to reproduce

1. Place an Accordion widget on a popup page.
2. Configure one or more groups to be initially expanded.
3. Open the popup.
4. Observe that the expand/collapse icon indicates an expanded group, but the content is not visible.
5. Inspect the DOM.

Example state after opening the popup:

contentHeight: 127
scrollHeight: 127
wrapperHeight: 0
inlineHeight: "0px"
aria-expanded: true

Disabling Animate does not change the behavior.

Expected behavior

An initially expanded Accordion group should render with its content visible when the popup opens.

Actual behavior

The Accordion content is rendered correctly, but the wrapper remains at an inline height of 0px.

Investigation

Accordion 2.3.6 uses a ResizeObserver on the inner content and updates the wrapper height using:

contentWrapperRef.current.style.height =
    `${contentRef.current.getBoundingClientRect().height}px`;

The problem appears to be a timing issue during popup initialization.

The likely sequence is:

Accordion renders inside popup
→ group state is correctly initialized as expanded
→ ResizeObserver callback runs while popup is still opening / not fully laid out
→ getBoundingClientRect().height returns 0
→ Accordion writes height: 0px to the wrapper
→ popup finishes opening
→ inner content now has its correct height
→ no new content-box resize occurs
→ ResizeObserver does not update the wrapper again
→ height remains 0px

This was confirmed by checking the group after the popup had fully opened:

contentHeight: 127
scrollHeight: 127
wrapperHeight: 0
inlineHeight: "0px"

An actual content-box size change was then forced on the inner content:

const wrapper = document.querySelector(
    '#Accordion42AccordionGroup1ContentWrapper'
);

const content = wrapper.querySelector(
    '.widget-accordion-group-content'
);

content.style.minHeight =
    `${content.getBoundingClientRect().height + 50}px`;

After this change, the Accordion's ResizeObserver fired again and correctly updated the wrapper height.

This confirms that the observer itself is working. The issue is that the initial height is measured incorrectly during popup rendering, and the later popup layout change does not trigger another resize event for the observed content.

Suspected root cause

The Accordion uses getBoundingClientRect().height inside a debounced ResizeObserver callback instead of relying on the size reported by the observer itself.

During popup opening, getBoundingClientRect() can temporarily return 0 because of the popup's rendering, visibility, or transform state, even though the content's actual layout size becomes non-zero shortly afterward.

Since the content's CSS content-box size may not change when the popup becomes fully visible, no second ResizeObserver event is generated, leaving the stale 0px inline height in place.

Additional notes

• The issue also occurs with Animate = No, because the ResizeObserver logic runs independently of the animation setting.
• The relevant ResizeObserver implementation appears unchanged across Accordion versions 2.3.3 through 2.3.6.
• This therefore does not appear to be a regression introduced specifically in version 2.3.6.

Screenshot