feat: allow users to force hide scrollbars

[lofcz]

Thanks for OverlayScrollbars, long time user here.. (I know OSS maintainers hear this a lot first hand, but still).

Problem

In a split layout, I animate the chat column with transforms, then switch to flex. On desktop, with scrollbars.autoHide: "move", the pointer often crosses the moving host during the animation. That drives the normal pointer/scroll auto-hide logic. When the animation ends and layout updates, the scrollbar can flash visible for a moment even if the cursor is no longer over the pane, it’s especially noticeable with the default opacity transition on .os-scrollbar.

2026-03-20.11-09-35.mp4

Proposed Solution

Add an imperative instance method so embedders can temporarily force-hide scrollbars and ignore the usual auto-show sensor paths until they call the method again to restore normal behavior:

export interface OverlayScrollbars {
+  scrollbarsHidden(hidden: boolean): void;
}

When hidden === true, the instance should keep overlay scrollbars in the hidden state across internal update() cycles (e.g. after resize) so layout changes don’t briefly apply os-scrollbar-visible without the matching auto-hide hidden state.