Function selectionFocusMoveForKey

src/components/ScrollKeybindingHandler.tsx:132–141

(key: Key)

 * preserves (modified nav). Returns null for non-extend keys.
 */
export function selectionFocusMoveForKey(key: Key): FocusMove | null {
  if (!key.shift || key.meta) return null;
  if (key.leftArrow) return 'left';
  if (key.rightArrow) return 'right';
  if (key.upArrow) return 'up';
  if (key.downArrow) return 'down';
  if (key.home) return 'lineStart';
  if (key.end) return 'lineEnd';
  return null;
}
export type WheelAccelState = {
  time: number;
  mult: number;