Overlay scrollbars via OverlayScrollbars

overflow: overlay is dead in current Chromium, so an appearing native
scrollbar shifted the layout again. OverlayScrollbars replaces it with
floating auto-hiding scrollbars that never reserve space (native window
scroll APIs unaffected on a body target). Themed via --os-* variables in
pagerite.css; the html scrollbar-color styling stays as no-JS fallback.
(package-lock.json is gitignored.)

AGENTS.md

@@ -119,6 +119,10 @@ not for the public pages. See `docs/design-principles.md` for the design.
   - `frontend/src/` — the Vue editor and public-page entries.
     - `main.js` — Vue editor app entry, mounts PageEditor/SiteEditor.
     - `pagerite.js` — public page entry; runs fetch-navigation, scroll-reveal,
+      OverlayScrollbars on `document.body` (floating, auto-hiding scrollbars
+      that never reserve layout space or shift the page when appearing;
+      native scroll APIs like `window.scrollTo` keep working; themed via the
+      `--os-*` variables in pagerite.css),
       brand shrink-to-fit (the themed size is the maximum; JS reduces the
       font-size so a long brand or narrow viewport still fits one line),
       code copy buttons, and the auth check. It first probes `GET /auth/api/settings`

frontend/package.json

@@ -18,6 +18,7 @@
     "@codemirror/view": "^6.43.8",
     "@lezer/highlight": "^1.2.3",
     "codemirror": "^6.0.2",
+    "overlayscrollbars": "^2.16.0",
     "paskia": "file:../../paskia/paskia-js",
     "transliteration": "^2.6.1",
     "vue": "^3.5.26",

frontend/src/assets/pagerite.css

@@ -67,31 +67,22 @@
 
 html {
   scroll-behavior: smooth;
-  /* Overlay scrollbars where supported (Chromium/WebKit, deprecated but
-     working): a scrollbar that appears covers a strip of the layout instead
-     of shifting it, and no space is reserved while absent — so 100vw always
-     equals the body width and .wide breakout margins stay exact. Ignored by
-     Firefox (unknown value), which keeps its thin classic scrollbar. */
-  overflow-y: overlay;
-  /* Lightweight scrollbar styling: thin, theme-muted thumb on a transparent
-     track where supported (Firefox: the two properties; WebKit: the rules
-     below). Full restyling deliberately not attempted. */
+  /* Native-scrollbar fallback styling (JS off or before pagerite.js runs):
+     thin, theme-muted thumb on a transparent track. With JS the scrollbars
+     are replaced by OverlayScrollbars (see pagerite.js) — floating,
+     auto-hidden scrollbars styled by the --os-* variables below, so they
+     never reserve layout space or shift the page when appearing. */
   scrollbar-width: thin;
   scrollbar-color: color-mix(in srgb, var(--muted) 45%, transparent) transparent;
 }
 
-::-webkit-scrollbar {
-  width: 0.5rem;
-  height: 0.5rem;
-}
-
-::-webkit-scrollbar-thumb {
-  background: color-mix(in srgb, var(--muted) 45%, transparent);
-  border-radius: 0.25rem;
-}
-
-::-webkit-scrollbar-track {
-  background: transparent;
+.os-scrollbar {
+  --os-size: 0.5rem;
+  --os-thumb-bg: color-mix(in srgb, var(--muted) 45%, transparent);
+  --os-thumb-hover-bg: color-mix(in srgb, var(--muted) 65%, transparent);
+  --os-thumb-active-bg: var(--muted);
+  --os-track-bg: transparent;
+  --os-thumb-border-radius: 0.25rem;
 }
 
 body {

frontend/src/pagerite.js

@@ -5,8 +5,20 @@
 // Also: scroll-reveal effects and code copy buttons. These need no
 // support from the article itself and are re-applied after each swap.
 import { showAuthIframe } from 'paskia'
+import { OverlayScrollbars } from "overlayscrollbars";
+import "overlayscrollbars/overlayscrollbars.css";
 
 (() => {
+  // Overlay scrollbars: the native kind reserves a strip of layout (or
+  // shifts the layout when it appears; overflow: overlay is dead), so we
+  // replace it with floating ones that cover content instead. The body
+  // remains the native viewport scroller — window scroll events, scrollY
+  // and scroll restoration are unaffected; only the scrollbar UI is
+  // custom. Theme variables are in pagerite.css.
+  OverlayScrollbars(document.body, {
+    scrollbars: { autoHide: "scroll" },
+  });
+
   if (import.meta.env.DEV) {
     // In dev the base stylesheet is injected by Vite from JS (linking the
     // raw module would pull in its HMR wrapper). Theme and banner-design