docs: fixed sidebar in API reference (#5871)

www/packages/docs-ui/src/utils/get-scrolled-top.ts

@@ -0,0 +1,8 @@
+import { isElmWindow } from "./is-elm-window"
+
+export function getScrolledTop(elm?: Element | Window): number {
+  if (!elm) {
+    return 0
+  }
+  return isElmWindow(elm) ? elm.scrollY : elm.scrollTop
+}

www/packages/docs-ui/src/utils/index.ts

@@ -3,4 +3,6 @@ export * from "./capitalize"
 export * from "./check-sidebar-item-visibility"
 export * from "./dom-utils"
 export * from "./format-report-link"
+export * from "./get-scrolled-top"
+export * from "./is-elm-window"
 export * from "./swr-fetcher"

www/packages/docs-ui/src/utils/is-elm-window.ts

@@ -0,0 +1,3 @@
+export function isElmWindow(elm: unknown): elm is Window {
+  return typeof window !== "undefined" && elm === window
+}