mAi: #85 - sidebar scroll position persists across nav

Sidebar nav clicks trigger a full page reload, which rebuilds the
sidebar from scratch and snaps .sidebar-nav back to scrollTop=0.
Persist scrollTop to sessionStorage (paliad.sidebar.scroll) on every
scroll and restore on initSidebar(). Re-apply once after
/api/user-views resolves so the async layout shift doesn't leave the
user a few rows off.

sessionStorage scopes the value to the tab: Cmd-click / right-click
"open in new tab" still produces a fresh tab that starts at the top.

frontend/src/client/sidebar.ts

@@ -11,6 +11,13 @@ const WIDTH_KEY = "paliad-sidebar-width";
 const SIDEBAR_WIDTH_MIN = 180;
 const SIDEBAR_WIDTH_MAX = 480;
 const SIDEBAR_WIDTH_DEFAULT = 240;
+// Per-tab scroll position of the .sidebar-nav scroll container. Persisted
+// on every scroll event, restored on initSidebar() so a full-page nav
+// click doesn't bounce the user back to the top of a long sidebar
+// (Werkzeuge + projects + user views can easily overflow). sessionStorage
+// scopes it to the tab — opening a sidebar link in a new tab (Cmd-click)
+// starts that tab fresh at the top, which matches user expectation.
+const SCROLL_KEY = "paliad.sidebar.scroll";
 
 // toggleMobileSidebar opens or closes the slide-out drawer. Exposed so the
 // BottomNav menu slot can call it without duplicating the open/close
@@ -49,6 +56,23 @@ function applySidebarWidth(px: number): void {
   document.documentElement.style.setProperty("--sidebar-width", `${px}px`);
 }
 
+// readStoredScroll returns the persisted scrollTop or 0 when missing /
+// malformed. Bounds are checked at apply time against the actual
+// scrollHeight, so a stale value pointing past the current scroll range
+// is harmless (the browser clamps assignments to [0, max]).
+function readStoredScroll(): number {
+  const raw = sessionStorage.getItem(SCROLL_KEY);
+  if (raw === null) return 0;
+  const n = parseInt(raw, 10);
+  if (!Number.isFinite(n) || n < 0) return 0;
+  return n;
+}
+
+function applySidebarScroll(nav: HTMLElement, px: number): void {
+  if (px <= 0) return;
+  nav.scrollTop = px;
+}
+
 // migrateLegacyPinKey copies the pre-rebrand pin state into the new key on
 // first load and removes the stale entry. Drop this fallback once the rename
 // grace period is over.
@@ -79,6 +103,7 @@ export function initSidebar() {
   const sidebar = document.querySelector<HTMLElement>(".sidebar");
   if (!sidebar) return;
   initSidebarResize(sidebar);
+  initSidebarScrollRestore(sidebar);
 
   const pinBtn = sidebar.querySelector<HTMLButtonElement>(".sidebar-pin");
   const hamburger = document.querySelector<HTMLButtonElement>(".sidebar-hamburger");
@@ -293,6 +318,29 @@ function initSidebarResize(sidebar: HTMLElement): void {
   });
 }
 
+// initSidebarScrollRestore wires the .sidebar-nav scroll container to
+// sessionStorage so the user's scroll position survives a full-page
+// navigation (every sidebar link click is a real reload — see m/paliad#85).
+// Restore is synchronous on init so the first paint is already at the
+// right offset; the passive scroll listener persists subsequent moves.
+// reapplySidebarScroll() exists so callers that mutate sidebar content
+// async (initUserViewsGroup appending /api/user-views into the Ansichten
+// group) can nudge the scroll back to where it was after the layout shift.
+function initSidebarScrollRestore(sidebar: HTMLElement): void {
+  const nav = sidebar.querySelector<HTMLElement>(".sidebar-nav");
+  if (!nav) return;
+  applySidebarScroll(nav, readStoredScroll());
+  nav.addEventListener("scroll", () => {
+    sessionStorage.setItem(SCROLL_KEY, String(nav.scrollTop));
+  }, { passive: true });
+}
+
+function reapplySidebarScroll(): void {
+  const nav = document.querySelector<HTMLElement>(".sidebar .sidebar-nav");
+  if (!nav) return;
+  applySidebarScroll(nav, readStoredScroll());
+}
+
 // Changelog badge — fetches the count of entries newer than the locally
 // stored "last seen" stamp and renders a dot + number on the Neuigkeiten
 // link. Skipped on the changelog page itself because changelog.ts stamps
@@ -432,6 +480,11 @@ function initUserViewsGroup(): void {
       for (const view of views) {
         items.appendChild(renderUserViewItem(view, currentPath));
       }
+      // The synchronous restore in initSidebarScrollRestore() happened
+      // before these views were appended, so a saved scrollTop that
+      // pointed below the Ansichten group would now sit on the wrong
+      // row. Re-apply once the layout has stabilised.
+      reapplySidebarScroll();
       // After rendering, kick off count refresh for views that opted in.
       for (const view of views) {
         if (view.show_count) {