perf(dashboard): slide live EPG progress fills with a compositor transform

The live-programme bars on channel rail cards and the hero animated their
width over 0.4 s whenever the 30 s live-EPG tick moved them. Width is a
layout property, so each tick re-laid out the whole document for about 24
frames, also while the window was minimized. Slide a full-width fill with
translateX driven by a --live-progress custom property instead; Chromium
runs that transition on the compositor. Reduced motion drops it.

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

.changes/dashboard-live-progress-compositor.md

@@ -0,0 +1,8 @@
+---
+type: perf
+area: dashboard
+---
+
+Live programme progress bars on the dashboard now advance without redrawing
+the whole page, so a dashboard left open with recent live channels uses less
+CPU and GPU.

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

@@ -141,7 +141,7 @@
                         aria-valuemax="100"
                         [attr.aria-valuenow]="slide.progress"
                     >
-                        <i [style.width.%]="slide.progress"></i>
+                        <i [style.--hero-progress]="slide.progress"></i>
                     </span>
                 }
 

libs/workspace/dashboard/feature/src/lib/rails/dashboard-hero.component.scss

@@ -339,10 +339,15 @@
 
     i {
         display: block;
+        width: 100%;
         height: 100%;
         border-radius: inherit;
         background: var(--app-selection-color, #78adff);
-        transition: width 0.4s ease;
+        // A full-width fill slid into place, neither a width nor animated: a
+        // live-EPG tick moves it by under one percent, so an animation was
+        // invisible yet cost a layout pass (width) or a burst of compositor
+        // frames (transform transition) on every tick.
+        transform: translateX(calc(var(--hero-progress, 0) * 1% - 100%));
     }
 
     &--live i {

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

@@ -126,7 +126,7 @@
                                     "
                                 >
                                     <i
-                                        [style.width.%]="
+                                        [style.--live-progress]="
                                             card.nowPlayingProgress ?? 0
                                         "
                                     ></i>

libs/workspace/dashboard/feature/src/lib/rails/dashboard-rail.component.scss

@@ -727,10 +727,19 @@
 
     i {
         display: block;
+        width: 100%;
         height: 100%;
         background: var(--app-live-color, #ff8f8f);
         border-radius: 2px;
-        transition: width 0.4s ease;
+        // A full-width fill slid into view, not an animated width: the
+        // transform transition runs on the compositor, so each live-EPG tick
+        // no longer re-lays out the whole page for every transition frame.
+        transform: translateX(calc(var(--live-progress, 0) * 1% - 100%));
+        transition: transform 0.4s ease;
+
+        @media (prefers-reduced-motion: reduce) {
+            transition: none;
+        }
     }
 
     &--idle i {

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

@@ -335,6 +335,26 @@ describe('DashboardRailComponent', () => {
             expect(visible.at(-1)).toEqual(['a', 'b']);
         });
 
+        it('positions the live progress fill through a custom property, never its width', async () => {
+            // Animating width re-lays out the page for every frame of the
+            // transition on each EPG tick; the stylesheet slides the fill with
+            // a compositor transform driven by this property instead.
+            installObservers(false);
+            const { fixture } = await render([
+                card({ id: 'known', contentType: 'live', nowPlayingProgress: 37.5 }),
+                card({ id: 'unknown', contentType: 'live' }),
+            ]);
+            const fills = Array.from(
+                (fixture.nativeElement as HTMLElement).querySelectorAll<HTMLElement>(
+                    '.rail__channel-progress i'
+                )
+            );
+            expect(fills).toHaveLength(2);
+            expect(fills[0].style.getPropertyValue('--live-progress')).toBe('37.5');
+            expect(fills[1].style.getPropertyValue('--live-progress')).toBe('0');
+            expect(fills.map((fill) => fill.style.width)).toEqual(['', '']);
+        });
+
         it('shows the placeholder only while a live card is pending its first answer', async () => {
             installObservers(false);
             const { fixture } = await render([