Function applyModalPagerAction

src/components/ScrollKeybindingHandler.tsx:969–1011

(s: ScrollBoxHandle, act: ModalPagerAction | null, onBeforeJump: (delta: number) => void)

 * shift anchor+focus) instead of clearing it. Exported for testing.
 */
export function applyModalPagerAction(s: ScrollBoxHandle, act: ModalPagerAction | null, onBeforeJump: (delta: number) => void): boolean | null {
  switch (act) {
    case null:
      return null;
    case 'lineUp':
    case 'lineDown':
      {
        const d = act === 'lineDown' ? 1 : -1;
        onBeforeJump(d);
        return jumpBy(s, d);
      }
    case 'halfPageUp':
    case 'halfPageDown':
      {
        const half = Math.max(1, Math.floor(s.getViewportHeight() / 2));
        const d = act === 'halfPageDown' ? half : -half;
        onBeforeJump(d);
        return jumpBy(s, d);
      }
    case 'fullPageUp':
    case 'fullPageDown':
      {
        const page = Math.max(1, s.getViewportHeight());
        const d = act === 'fullPageDown' ? page : -page;
        onBeforeJump(d);
        return jumpBy(s, d);
      }
    case 'top':
      onBeforeJump(-(s.getScrollTop() + s.getPendingDelta()));
      s.scrollTo(0);
      return false;
    case 'bottom':
      {
        const max = Math.max(0, s.getScrollHeight() - s.getViewportHeight());
        onBeforeJump(max - (s.getScrollTop() + s.getPendingDelta()));
        // Eager-write scrollTop before scrollToBottom — same double-shift
        // fix as scroll:bottom and jumpBy's max branch.
        s.scrollTo(max);
        s.scrollToBottom();
        return true;
      }
  }
}