perf(dashboard): hold rail skeletons back so empty rails stop shifting the page

On every launch with sources the dashboard shifted by about 0.23 (the
"good" CLS threshold is 0.1). The rails render as soon as their own data
arrives, and each loading rail showed a 328 px skeleton immediately. On a
normal profile the live-favorites and recent-content sources resolve empty
15-20 ms later, so their skeletons flashed and collapsed and every rail
below jumped up by about 360 px. J1 never saw it: its layout-shift window
ends at the first card, which is painted just before the collapse.

Rail skeletons now wait out a 300 ms grace period (createRailSkeletonGrace)
and appear only for a rail still loading after it; the hero keeps its
immediate skeleton because it reserves the top of the page. Recorded over
three renderer reloads of a seeded profile, the dashboard's layout shift
drops from 0.219-0.234 to 0.0004, with the real rails painted at the same
time as before. Plan thread C5, journey J1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/dashboard-no-rail-jump.md

@@ -0,0 +1,9 @@
+---
+type: perf
+area: dashboard
+---
+
+The dashboard no longer jumps when it opens: placeholder rails that turned out
+to be empty used to flash and collapse, pushing the rails below them up the
+page. Rails now appear once, in place, and a placeholder shows only when a rail
+takes noticeably long to load.

docs/architecture/iptvnator-ui-guidelines.md

@@ -542,6 +542,20 @@ mirrors the row/card geometry it precedes (see Channel List Item and Cover
 Grids). The unified Favorites/Recent page gates this on `isLoading`, set only
 while its item list is empty.
 
+### Pages of independently loading blocks: delayed skeletons
+
+The dashboard renders each rail as soon as its own data arrives, and several
+rails resolve empty on a normal profile. A per-rail skeleton shown
+immediately therefore flashed for a few tens of milliseconds and collapsed,
+pulling every rail below it upwards (a layout shift of about 0.23 on each
+launch with sources). Rail skeletons wait out a short grace period
+(`DASHBOARD_RAIL_SKELETON_GRACE_MS`, 300 ms, in
+`libs/workspace/dashboard/feature/src/lib/rails/dashboard-skeleton-grace.ts`)
+and appear only for a rail that is still loading after it. The top block
+(the dashboard hero) keeps its immediate skeleton: it reserves the space
+above everything else, where a late insertion would push the whole page down.
+Use the same rule for any page that stacks independently loading blocks.
+
 ### Reload with content on screen: non-destructive indicator
 
 A reload of a list that is already rendered (the collection page's

libs/workspace/dashboard/feature/src/lib/rails/dashboard-skeleton-grace.spec.ts

@@ -0,0 +1,63 @@
+import {
+    DestroyRef,
+    EnvironmentInjector,
+    createEnvironmentInjector,
+} from '@angular/core';
+import { TestBed } from '@angular/core/testing';
+import {
+    DASHBOARD_RAIL_SKELETON_GRACE_MS,
+    createRailSkeletonGrace,
+} from './dashboard-skeleton-grace';
+
+describe('createRailSkeletonGrace', () => {
+    beforeEach(() => jest.useFakeTimers());
+    afterEach(() => jest.useRealTimers());
+
+    function createInScope(delayMs?: number) {
+        const injector = createEnvironmentInjector(
+            [],
+            TestBed.inject(EnvironmentInjector)
+        );
+        const grace = injector.runInContext(() =>
+            createRailSkeletonGrace(delayMs)
+        );
+        return { grace, injector };
+    }
+
+    it('keeps skeletons hidden while rails that load quickly resolve', () => {
+        const { grace } = createInScope();
+
+        jest.advanceTimersByTime(DASHBOARD_RAIL_SKELETON_GRACE_MS - 1);
+
+        expect(grace()).toBe(false);
+    });
+
+    it('allows skeletons once a rail keeps loading past the grace period', () => {
+        const { grace } = createInScope();
+
+        jest.advanceTimersByTime(DASHBOARD_RAIL_SKELETON_GRACE_MS);
+
+        expect(grace()).toBe(true);
+    });
+
+    it('clears its timer when the dashboard is destroyed', () => {
+        const { grace, injector } = createInScope();
+
+        injector.destroy();
+        jest.advanceTimersByTime(DASHBOARD_RAIL_SKELETON_GRACE_MS * 2);
+
+        expect(grace()).toBe(false);
+        expect(jest.getTimerCount()).toBe(0);
+    });
+
+    it('treats a zero delay as elapsed without scheduling a timer', () => {
+        const { grace } = createInScope(0);
+
+        expect(grace()).toBe(true);
+        expect(jest.getTimerCount()).toBe(0);
+    });
+
+    it('needs an injection context', () => {
+        expect(() => createRailSkeletonGrace()).toThrow();
+    });
+});

libs/workspace/dashboard/feature/src/lib/rails/dashboard-skeleton-grace.ts

@@ -0,0 +1,31 @@
+import { DestroyRef, inject, signal, type Signal } from '@angular/core';
+
+/**
+ * How long a dashboard rail may keep loading before its skeleton appears.
+ *
+ * On a warm profile the local rails resolve within a few tens of
+ * milliseconds of the first render, and several of them resolve empty. An
+ * immediate skeleton for each loading rail therefore flashed and then
+ * collapsed, pulling every rail below it upwards: a layout shift of about
+ * 0.23 on every launch with sources (the "good" CLS threshold is 0.1).
+ * Holding the skeletons back for a short grace period means a fast rail
+ * appears once, in place, and a genuinely slow one (TMDB, a large portal)
+ * still gets its placeholder. The hero skeleton is not delayed: it reserves
+ * the top of the page, where a late insertion would push everything down.
+ */
+export const DASHBOARD_RAIL_SKELETON_GRACE_MS = 300;
+
+/**
+ * A signal that turns true once the grace period has elapsed. Must be
+ * created in an injection context; the timer is cleared on destroy.
+ */
+export function createRailSkeletonGrace(
+    delayMs = DASHBOARD_RAIL_SKELETON_GRACE_MS
+): Signal<boolean> {
+    const elapsed = signal(delayMs <= 0);
+    if (delayMs > 0) {
+        const timer = setTimeout(() => elapsed.set(true), delayMs);
+        inject(DestroyRef).onDestroy(() => clearTimeout(timer));
+    }
+    return elapsed.asReadonly();
+}

libs/workspace/dashboard/feature/src/lib/rails/workspace-dashboard-rails.component.html

@@ -238,7 +238,7 @@
                     liveEpg.setVisibleCards('favorites', $event)
                 "
             />
-        } @else if (showLiveFavoritesSkeleton()) {
+        } @else if (railSkeletonsVisible() && showLiveFavoritesSkeleton()) {
             <section
                 class="rails-page__skeleton-rail"
                 aria-hidden="true"
@@ -290,7 +290,7 @@
             />
         }
 
-        @if (showRecentContentSkeleton()) {
+        @if (railSkeletonsVisible() && showRecentContentSkeleton()) {
             <section
                 class="rails-page__skeleton-rail"
                 aria-hidden="true"
@@ -316,7 +316,11 @@
                 [testId]="'dashboard-recent-sources-rail'"
                 (actionSelected)="onSourceActionSelected($event)"
             />
-        } @else if (dashboardRails().recentSources && !data.playlistsLoaded()) {
+        } @else if (
+            railSkeletonsVisible() &&
+            dashboardRails().recentSources &&
+            !data.playlistsLoaded()
+        ) {
             <section
                 class="rails-page__skeleton-rail"
                 aria-hidden="true"
@@ -345,6 +349,7 @@
                 [testId]="'dashboard-xtream-recently-added-rail'"
             />
         } @else if (
+            railSkeletonsVisible() &&
             dashboardRails().xtreamRecentlyAdded &&
             xtreamPlaylistCount() > 0 &&
             data.xtreamRecentlyAddedLoading()
@@ -375,6 +380,7 @@
                 [testId]="'dashboard-tmdb-recommendations-rail'"
             />
         } @else if (
+            railSkeletonsVisible() &&
             dashboardRails().tmdbRecommendations &&
             recommendationsService.loading()
         ) {
@@ -401,7 +407,9 @@
                 [testId]="'dashboard-tmdb-trending-rail'"
             />
         } @else if (
-            dashboardRails().tmdbTrending && trendingService.loading()
+            railSkeletonsVisible() &&
+            dashboardRails().tmdbTrending &&
+            trendingService.loading()
         ) {
             <section
                 class="rails-page__skeleton-rail"

libs/workspace/dashboard/feature/src/lib/rails/workspace-dashboard-rails.component.ts

@@ -55,6 +55,7 @@ import {
     resolveSourceExpiryBadge,
     SOURCE_EXPIRY_TICK_MS,
 } from '@iptvnator/workspace/dashboard/data-access';
+import { createRailSkeletonGrace } from './dashboard-skeleton-grace';
 import type { DashboardHeroTmdbExtras } from './dashboard-hero-tmdb.service';
 import { DashboardHeroTmdbService } from './dashboard-hero-tmdb.service';
 import { DashboardRailComponent } from './dashboard-rail.component';
@@ -141,6 +142,8 @@ export class WorkspaceDashboardRailsComponent {
     readonly isElectron = this.runtime.isElectron;
 
     readonly skeletonSlots = SKELETON_CARDS_PER_RAIL;
+    /** Rail skeletons wait out a short grace period; see the helper. */
+    readonly railSkeletonsVisible = createRailSkeletonGrace();
     readonly skeletonRails = SKELETON_RAILS;
     readonly liveRailTitleKeyForSource = liveRailTitleKeyForSource;
     readonly failedHeroImages = signal<Record<string, true>>({});