fix(scroll): reference-count locks so stacked locks release correctly

[blvdmitry]

Problem

lockScroll short-circuited when the container was already locked:

// Already locked so no need to lock again and trigger the callback
if (container.style.overflow === "hidden") return;

This returns undefined, so a second locker (e.g. a modal opening a dropdown, or two modals both locking the document) gets no unlock function and its callback never fires. And because there's no tracking of who holds the lock, releasing the first lock restores the scroll while the second is still open — the page becomes scrollable under an open overlay, and the second overlay has no way to unlock.

Fix

Reference-count active locks per container (via WeakMap): apply the style changes only for the first lock, restore only when the last lock is released, and always return a valid, idempotent unlock function.

const lockCounts = new WeakMap<HTMLElement, number>();
const lockResets = new WeakMap<HTMLElement, () => void>();
// ...
if (currentCount === 0) { /* apply lock, store reset */ }
lockCounts.set(container, currentCount + 1);
// unlock(): decrement; when it hits 0, run the stored reset

Behavioural note: lockScroll now always returns an unlock function (previously it could return undefined). Callers that did unlock?.() are unaffected.

Verification

Logic simulated:

after 2 locks: applied=1 reset=0   (styles applied once)
after unlock A: reset=0            (B still open -> still locked)
after unlock B: reset=1            (last unlock -> restored)
double unlock A: reset=1           (idempotent)

• tsc --noEmit (utilities) → 0 errors. oxlint → clean.

How to test

1. Open a Modal, then from within it open a Dropdown/another Modal (both lock the document).
2. Close them in either order.
3. Before: closing the first re-enables page scroll while the second is still open; the second may never restore scroll. After: scroll stays locked until the last overlay closes, then restores.

Found during a full package bug review. Complementary to the StyleCache-scoping and iOS-guard PRs in the same cluster.

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Generated by Claude Code

[github-actions]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
Library / JS	48.71 KB (0%)	975 ms (0%)	312 ms (+7.44% 🔺)	1.3 s
Library / CSS	22.18 KB (0%)	444 ms (0%)	0 ms (+100% 🔺)	444 ms
Theming / JS	7.81 KB (0%)	157 ms (0%)	257 ms (+442.13% 🔺)	413 ms
Theming with a default theme definition / JS	8.65 KB (0%)	174 ms (0%)	49 ms (+2.99% 🔺)	222 ms