feat(renderer): useScrollSync hook (60fps throttling, ratio calc)

src/renderer/hooks/use-scroll-sync.ts

@@ -0,0 +1,28 @@
+import { useCallback, useRef } from 'react';
+
+interface Options {
+  onEditorScroll?: (ratio: number) => void;
+  onPreviewScroll?: (ratio: number) => void;
+}
+
+export function useScrollSync(opts: Options) {
+  const FRAME_MS = 1000 / 60;
+  const lastTick = useRef(-FRAME_MS);
+
+  const handleEditorScroll = useCallback((evt: React.UIEvent<HTMLElement>) => {
+    const target = evt.currentTarget;
+    const ratio = target.scrollTop / Math.max(target.scrollHeight - target.clientHeight, 1);
+    const now = performance.now();
+    if (now - lastTick.current < FRAME_MS) return;
+    lastTick.current = now;
+    opts.onEditorScroll?.(ratio);
+  }, [opts]);
+
+  const handlePreviewScroll = useCallback((evt: React.UIEvent<HTMLElement>) => {
+    const target = evt.currentTarget;
+    const ratio = target.scrollTop / Math.max(target.scrollHeight - target.clientHeight, 1);
+    opts.onPreviewScroll?.(ratio);
+  }, [opts]);
+
+  return { handleEditorScroll, handlePreviewScroll };
+}