fix(epg): estimate the ribbon window once per channel or mount

The live estimate followed the centred day and the 30 s now tick while the
ribbon was not yet scrolled: a small scroll across midnight re-centred the
window on the next day's noon and left the visible range empty, and the
host width was re-read (a forced layout) on every tick.

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

libs/ui/epg/src/lib/epg-timeline/epg-timeline-window.controller.spec.ts

@@ -331,6 +331,29 @@ describe('EpgTimelineComponent ribbon windowing', () => {
         expect(renderedTitles()).not.toContain('Slot 0');
     });
 
+    it('keeps the window where the ribbon is when a small scroll commits another day', () => {
+        // A scroll under the re-window step can still cross midnight; the
+        // committed day must not re-centre the window on its noon.
+        render(mockSchedule());
+        const tomorrow = new Date(Date.now() + 24 * 60 * TIMELINE_MINUTE_MS);
+        const pad = (n: number) => String(n).padStart(2, '0');
+        component.viewDayKey.set(
+            `${tomorrow.getFullYear()}-${pad(tomorrow.getMonth() + 1)}-${pad(tomorrow.getDate())}`
+        );
+        fixture.detectChanges();
+
+        expect(renderedTitles()).toContain('Slot 96');
+    });
+
+    it('does not follow the now tick away from an unscrolled ribbon', () => {
+        render(mockSchedule());
+
+        component.nowMs.set(Date.now() + 12 * 60 * TIMELINE_MINUTE_MS);
+        fixture.detectChanges();
+
+        expect(renderedTitles()).toContain('Slot 96');
+    });
+
     it('starts over around now when another channel loads', () => {
         render(mockSchedule());
         scrollRibbonTo(blockLeftPx('Slot 230'));

libs/ui/epg/src/lib/epg-timeline/epg-timeline-window.controller.ts

@@ -5,6 +5,7 @@ import {
     inject,
     linkedSignal,
     Signal,
+    untracked,
 } from '@angular/core';
 import { EpgProgram } from '@iptvnator/shared/interfaces';
 import { getTodayEpgDateKey, parseEpgDateKey } from '../epg-date';
@@ -154,13 +155,15 @@ export class TimelineWindowController {
                 left.ribbon === right.ribbon && left.key === right.key,
         }
     );
-    private readonly measured = linkedSignal<unknown, TimelineViewport | null>({
+    /**
+     * Estimated once per channel or ribbon mount, then moved only by the
+     * scroll and resize measurements: a live estimate would follow the 30 s
+     * now tick and the centred day away from what the ribbon shows.
+     */
+    private readonly viewport = linkedSignal<unknown, TimelineViewport>({
         source: this.identity,
-        computation: () => null,
+        computation: () => untracked(() => this.initialViewport()),
     });
-    private readonly viewport = computed(
-        () => this.measured() ?? this.initialViewport()
-    );
     private readonly range = computed(() =>
         timelineWindowRange(this.viewport(), this.ctx.axis(), this.ctx.scale())
     );
@@ -221,7 +224,7 @@ export class TimelineWindowController {
         const widthPx = this.scroller()?.clientWidth ?? 0;
         const current = this.viewport();
         if (widthPx > current.widthPx) {
-            this.measured.set({ centreMs: current.centreMs, widthPx });
+            this.viewport.set({ centreMs: current.centreMs, widthPx });
         }
     }
 
@@ -239,7 +242,7 @@ export class TimelineWindowController {
             scale
         );
         if (viewportNeedsWindow(this.viewport(), next, scale)) {
-            this.measured.set(next);
+            this.viewport.set(next);
         }
     }