Absolute positioning

Not a collapse, but a dropdown.

Live demo

Just position it absolutely.

There is some dropdown content here. We give it a maximum width.

Markup

HTML 
<div class="absolute-panel-wrapper">
	<button aria-controls="absolute-panel">Toggle dropdown</button>
	<div data-panel id="absolute-panel">
		<div class="panel-wrapper">
			<p>There is some dropdown content here. We give it a maximum width.</p>
		</div>
	</div>
</div>

Styling

The panel is styled like a dropdown, with a shadow and a white background. The animation timing is tweaked to give it a snappier feel than the default Panel open.

--ps-open-transition-extra is used to style the data-panel itself. Note that we do not give it a real border, but a box-shadow that looks like a border, because even in a closed state, a border will be visible.

CSS 
.absolute-panel-wrapper {
	position: relative;
	width: max-content;
}

/* Keep the content at a fixed width so it doesn’t reflow while animating. */
#absolute-panel .panel-wrapper {
	padding: 1rem;
	flex-shrink: 0;
	width: 220px;
}

#absolute-panel {
	--ps-open-timing:  cubic-bezier(0.34, 1.56, 0.64, 1);
	--ps-close-timing: cubic-bezier(0.36, 0, 0.66, -0.56);

	/* Animate box-shadow, width and opacity alongside the height. */
	--ps-open-transition-extra:  box-shadow var(--ps-open-speed) var(--ps-open-timing), width var(--ps-open-speed) var(--ps-open-timing), opacity var(--ps-open-speed) var(--ps-open-timing);
	--ps-close-transition-extra: box-shadow var(--ps-close-speed) var(--ps-close-timing), width var(--ps-close-speed) var(--ps-close-timing), opacity var(--ps-close-speed) var(--ps-close-timing);

	position: absolute;
	right: 0;        /* right edges aligned */
	z-index: 10;
	width: 100%;     /* closed: as wide as the trigger */

	border-radius: 8px;
	background: white;
	box-shadow: inset 0 0 0 0px var(--special-normal), 0 0 0 var(--special-shadow-dark);
	opacity: 0;
	display: flex;
	justify-content: flex-end;
}

#absolute-panel.is-opening,
#absolute-panel.is-open {
	box-shadow: inset 0 0 0 2px var(--special-normal), 0 8px 20px var(--special-shadow-dark);
	width: 220px;    /* open: out to full width */
	opacity: 1;
}