fix(collapsible): open the panel to the content's exact size

.changeset/collapse-fractional-height.md

@@ -0,0 +1,5 @@
+---
+'@human-kit/ui': patch
+---
+
+A collapsible panel opens to its content's exact size instead of the whole pixels `scrollHeight` rounds to. Content 57.5px tall used to open a 58px panel, half a pixel taller than the same page rendered on the server, so everything under the panel moved when the page hydrated.

packages/ui/src/lib/primitives/collapse-transition.svelte.test.ts

@@ -3,6 +3,7 @@ import { render } from 'vitest-browser-svelte';
 import { userEvent } from 'vitest/browser';
 
 import CollapseTransitionTest from './collapse-transition-test.svelte';
+import { exactSize } from './collapse-transition.svelte';
 
 function panel() {
 	return document.querySelector('[data-testid="panel"]');
@@ -42,3 +43,14 @@ describe('createCollapseTransition', () => {
 		expect(panel()?.getAttribute('data-status')).toBe('idle');
 	});
 });
+
+describe('exactSize', () => {
+	it('keeps the half pixel the scroll size rounds away', () => {
+		expect(exactSize(58, 57.5)).toBe(57.5);
+		expect(exactSize(57, 57.4)).toBe(57.4);
+	});
+
+	it('opens to the scroll size when the content overflows its box', () => {
+		expect(exactSize(80, 57.5)).toBe(80);
+	});
+});

packages/ui/src/lib/primitives/collapse-transition.svelte.ts

@@ -20,6 +20,19 @@ export type CollapseTransition = {
 	setPanel: (panel: HTMLElement | null, content?: HTMLElement | null) => void;
 };
 
+/**
+ * The content's size without the rounding `scrollHeight`/`scrollWidth` apply.
+ *
+ * Those are whole pixels: content 57.5px tall measures 58, so an open panel came out half a pixel
+ * taller than the same content laid out without the variable (on the server, before hydration),
+ * and everything under it moved when the page hydrated. Within a pixel of the laid-out box the
+ * scroll size is only that rounding, so the box's own fractional size is the answer; beyond it the
+ * content really overflows its box, and the scroll size is what the panel has to open to.
+ */
+export function exactSize(scrollSize: number, boxSize: number): number {
+	return Math.abs(scrollSize - boxSize) < 1 ? boxSize : scrollSize;
+}
+
 /**
  * Drives a collapsible panel's enter/exit transitions the way Base UI's Collapsible does:
  * - keeps the content mounted for the full exit animation, then unmounts it (or, with
@@ -51,8 +64,8 @@ export function createCollapseTransition(
 	function measure() {
 		// Freeze the size while collapsing so the exit animates from the open height, not from 0.
 		if (!measured || status === 'ending') return;
-		height = measured.scrollHeight;
-		width = measured.scrollWidth;
+		height = exactSize(measured.scrollHeight, measured.getBoundingClientRect().height);
+		width = exactSize(measured.scrollWidth, measured.getBoundingClientRect().width);
 	}
 
 	function cancelRelease() {