fix(announcements): float the banner instead of occupying layout

frontend/ai.client/src/app/app.html

@@ -41,9 +41,7 @@
 
   <!-- Desktop controls (when sidenav collapsed) -->
   @if (sidenavService.isCollapsed() && !chromeHidden()) {
-    <div
-      class="hidden lg:flex fixed left-4 z-50 gap-2"
-      [style.top]="'calc(1rem + var(--announcement-banner-height, 0px))'">
+    <div class="hidden lg:flex fixed top-4 left-4 z-50 gap-2">
       <!-- Expand sidebar button -->
       <button
         type="button"
@@ -74,9 +72,7 @@
 
   <!-- Mobile controls (when header content hidden and sidenav not visible) -->
   @if (!headerService.showContent() && !sidenavService.isVisible() && !chromeHidden()) {
-    <div
-      class="flex lg:hidden fixed left-4 z-50 gap-2"
-      [style.top]="'calc(1rem + var(--announcement-banner-height, 0px))'">
+    <div class="flex lg:hidden fixed top-4 left-4 z-50 gap-2">
       <!-- Open sidenav button -->
       <button
         type="button"
@@ -111,12 +107,11 @@
     [class.lg:pl-72]="!sidenavService.isCollapsed() && !chromeHidden()"
     [class.lg:pl-0]="sidenavService.isCollapsed() || chromeHidden()"
     [class.artifact-pane-open]="artifactPanelOpen()">
-    <main class="flex h-dvh flex-col">
-      <!-- Ambient announcement strip (spec §D1). A flex child rather than an
-           overlay, so the scrolling content below reflows instead of hiding
-           under it. It publishes its measured height as
-           --announcement-banner-height; the fixed chat topnav and the
-           floating sidenav controls offset against that. -->
+    <main class="relative flex h-dvh flex-col">
+      <!-- Ambient announcement strip (spec §D1). Positioned `absolute` against
+           this `relative` main, so showing or dismissing it never reflows the
+           page. `relative` is load-bearing: without it the pill would anchor
+           to the viewport and drift out from under the sidenav padding. -->
       @if (showAnnouncementBanner()) {
         <app-announcement-banner />
       }

frontend/ai.client/src/app/components/announcement-banner/announcement-banner.component.spec.ts

@@ -28,30 +28,11 @@ function makeAnnouncement(overrides: Partial<Announcement> = {}): Announcement {
 describe('AnnouncementBannerComponent', () => {
   let bannerItem: ReturnType<typeof signal<Announcement | null>>;
   let ack: ReturnType<typeof vi.fn>;
-  let observed: Element[];
 
   beforeEach(() => {
     TestBed.resetTestingModule();
     bannerItem = signal<Announcement | null>(null);
     ack = vi.fn(async () => true);
-    observed = [];
-
-    // jsdom has no ResizeObserver. Stub one that records what it watches so
-    // the height-publishing path is actually exercised rather than skipped.
-    (globalThis as any).ResizeObserver = class {
-      constructor(private cb: ResizeObserverCallback) {}
-      observe(el: Element) {
-        observed.push(el);
-      }
-      disconnect() {}
-      unobserve() {}
-      emit(height: number) {
-        this.cb(
-          [{ contentRect: { height } } as unknown as ResizeObserverEntry],
-          this as unknown as ResizeObserver,
-        );
-      }
-    };
 
     // DI-token override rather than vi.mock, per house convention.
     TestBed.configureTestingModule({
@@ -63,7 +44,6 @@ describe('AnnouncementBannerComponent', () => {
 
   afterEach(() => {
     TestBed.resetTestingModule();
-    document.documentElement.style.removeProperty('--announcement-banner-height');
   });
 
   function create() {
@@ -204,29 +184,59 @@ describe('AnnouncementBannerComponent', () => {
     expect(link.getAttribute('rel')).toBe('noopener noreferrer');
   });
 
-  it('publishes its measured height so the fixed topnav can clear it', () => {
-    bannerItem.set(makeAnnouncement());
-    const fixture = create();
+  describe('overlays rather than occupying space', () => {
+    it('positions the host absolutely, so dismissing it cannot reflow the page', () => {
+      // The regression: the banner used to be a flex child of the shell's
+      // <main>, so showing or hiding it pulled the whole view up or down by
+      // its height. It must never participate in flow.
+      bannerItem.set(makeAnnouncement());
+      const fixture = create();
+      const host = fixture.nativeElement as HTMLElement;
+
+      expect(host.className).toContain('absolute');
+      expect(host.className).not.toContain('block');
+    });
 
-    expect(observed).toHaveLength(1);
-    expect(observed[0]).toBe(fixture.nativeElement);
-  });
+    it('lets clicks through the positioning strip to the chrome beneath', () => {
+      // The strip spans the full content width. Without this, it would
+      // swallow clicks aimed at the topnav and the sidebar buttons under it.
+      bannerItem.set(makeAnnouncement());
+      const fixture = create();
+      const host = fixture.nativeElement as HTMLElement;
 
-  it('clears the height variable on destroy, so nothing is left offset', () => {
-    bannerItem.set(makeAnnouncement());
-    const fixture = create();
-    expect(
-      document.documentElement.style.getPropertyValue(
-        '--announcement-banner-height',
-      ),
-    ).not.toBe('');
+      expect(host.className).toContain('pointer-events-none');
+      expect(strip(fixture)!.className).toContain('pointer-events-auto');
+    });
+
+    it('clears the topnav and the floating sidenav controls', () => {
+      // `top-16` puts the pill just below a chat route's fixed topnav, and
+      // below the shell's `top-4` sidebar buttons everywhere else — one
+      // constant instead of route awareness.
+      bannerItem.set(makeAnnouncement());
+      const fixture = create();
+      expect((fixture.nativeElement as HTMLElement).className).toContain('top-16');
+    });
 
-    fixture.destroy();
+    it('reads as a floating card, not a full-bleed strip', () => {
+      bannerItem.set(makeAnnouncement());
+      const fixture = create();
+      const pill = strip(fixture)!;
 
-    expect(
-      document.documentElement.style.getPropertyValue(
-        '--announcement-banner-height',
-      ),
-    ).toBe('');
+      expect(pill.className).toContain('rounded-2xl');
+      expect(pill.className).toContain('shadow-lg');
+      expect(pill.className).toContain('max-w-2xl');
+      // The old full-bleed look leaned on a bottom border instead.
+      expect(pill.className).not.toContain('border-b');
+    });
+
+    it('sets no layout variable on the document — nothing offsets against it', () => {
+      bannerItem.set(makeAnnouncement());
+      create();
+      expect(
+        document.documentElement.style.getPropertyValue(
+          '--announcement-banner-height',
+        ),
+      ).toBe('');
+    });
   });
 });

frontend/ai.client/src/app/components/announcement-banner/announcement-banner.component.ts

@@ -1,14 +1,10 @@
 import {
   ChangeDetectionStrategy,
   Component,
-  DestroyRef,
-  ElementRef,
   computed,
   effect,
   inject,
-  signal,
 } from '@angular/core';
-import { DOCUMENT } from '@angular/common';
 import { NgIcon, provideIcons } from '@ng-icons/core';
 import {
   heroCheckCircle,
@@ -45,12 +41,23 @@ import {
  * the POST fails, so a transient 500 cannot trap a user under an
  * undismissable strip.
  *
- * **It publishes its own height** as `--announcement-banner-height` on the
- * document root. The strip is a flex child of the shell's `<main>`, so the
- * scrolling content reflows on its own — but the chat topnav is
- * `position: fixed`, and would sit on top of the banner without that offset.
- * The value is measured rather than hardcoded because the line wraps on narrow
- * viewports.
+ * **It overlays rather than occupying space.** The host is positioned
+ * `absolute` inside the shell's `<main>`, so showing or dismissing it never
+ * reflows the page — an earlier version was a flex child, and dismissing it
+ * pulled the whole view up by its height. Overlaying also removes the reason
+ * three pieces of viewport-fixed chrome used to offset against a measured
+ * `--announcement-banner-height`: nothing has to move any more, so that
+ * variable, its `ResizeObserver`, and all three offsets are gone.
+ *
+ * `top-16` is not arbitrary. On a chat route it lands the pill immediately
+ * below the fixed topnav — the placement §D1 asks for — and everywhere else
+ * it clears the shell's floating sidenav controls, which sit at `top-4` and
+ * would otherwise be overlapped by a centred pill on any viewport narrow
+ * enough for the two to meet. One constant, no route awareness.
+ *
+ * The wrapper is `pointer-events-none` and only the pill itself takes events,
+ * so the full-width positioning strip cannot swallow clicks aimed at the
+ * topnav or the sidebar buttons underneath it.
  *
  * Body markdown is deliberately *not* rendered here: this surface is one line.
  * The full body lives in What's New, which is why `panel` is forced onto every
@@ -68,11 +75,16 @@ import {
       heroXMark,
     }),
   ],
-  host: { class: 'block' },
+  host: {
+    // The positioning strip. `pointer-events-none` here (and `auto` on the
+    // pill) keeps it from swallowing clicks meant for the chrome beneath.
+    class:
+      'pointer-events-none absolute inset-x-0 top-16 z-50 flex justify-center px-4',
+  },
   template: `
     @if (announcement(); as item) {
       <div
-        class="flex items-center gap-x-3 border-b px-4 py-2.5 sm:px-6"
+        class="pointer-events-auto flex w-full max-w-2xl items-center gap-x-3 rounded-2xl border px-4 py-2.5 shadow-lg sm:px-5"
         [class]="severityClass()"
         role="status"
         aria-live="polite"
@@ -112,12 +124,6 @@ import {
 })
 export class AnnouncementBannerComponent {
   private readonly announcements = inject(AnnouncementsService);
-  private readonly host: ElementRef<HTMLElement> = inject(ElementRef);
-  private readonly destroyRef = inject(DestroyRef);
-  private readonly document = inject(DOCUMENT);
-
-  /** The CSS custom property the shell's fixed chrome offsets against. */
-  private static readonly HEIGHT_VAR = '--announcement-banner-height';
 
   readonly announcement = computed<Announcement | null>(() =>
     this.announcements.bannerItem(),
@@ -175,42 +181,13 @@ export class AnnouncementBannerComponent {
     () => this.announcement()?.severity ?? 'info',
   );
 
-  /** Measured height of the strip, mirrored onto the document root. */
-  private readonly height = signal(0);
-
   constructor() {
     effect(() => {
       const item = this.announcement();
       if (!item || this.reportedSeen.has(item.announcement_id)) return;
       this.reportedSeen.add(item.announcement_id);
       void this.announcements.ack(item.announcement_id, 'seen', 'banner');
     });
-
-    effect(() => {
-      const px = this.height();
-      this.document.documentElement.style.setProperty(
-        AnnouncementBannerComponent.HEIGHT_VAR,
-        `${px}px`,
-      );
-    });
-
-    // `ResizeObserver` rather than a one-shot measurement: the line wraps when
-    // the viewport narrows or the artifact pane opens, and the fixed topnav
-    // has to follow it down.
-    if (typeof ResizeObserver !== 'undefined') {
-      const observer = new ResizeObserver(entries => {
-        const next = entries[0]?.contentRect.height ?? 0;
-        this.height.set(Math.round(next));
-      });
-      observer.observe(this.host.nativeElement);
-      this.destroyRef.onDestroy(() => observer.disconnect());
-    }
-
-    this.destroyRef.onDestroy(() => {
-      this.document.documentElement.style.removeProperty(
-        AnnouncementBannerComponent.HEIGHT_VAR,
-      );
-    });
   }
 
   protected onDismiss(): void {

frontend/ai.client/src/app/session/components/chat-container/chat-container.component.css

@@ -8,11 +8,7 @@
 /* Topnav wrapper for full-page mode */
 .chat-topnav-wrapper {
   position: fixed;
-  /* Sits directly below the announcement strip when one is showing. The strip
-     is a flex child of the shell's <main>, so the scroll container already
-     starts below it — but this bar is fixed and would otherwise cover it.
-     The variable is published (and cleared) by app-announcement-banner. */
-  top: var(--announcement-banner-height, 0px);
+  top: 0;
   left: 0;
   right: 0;
   z-index: 40;
@@ -53,13 +49,7 @@
 /* Empty state container for full-page mode */
 .chat-container-empty.full-page {
   position: fixed;
-  /* Not `inset: 0` — this overlay is viewport-fixed and opaque, so a flush
-     top would paint over the announcement strip that sits above the shell's
-     scroll container. Same offset the topnav takes. */
-  top: var(--announcement-banner-height, 0px);
-  right: 0;
-  bottom: 0;
-  left: 0;
+  inset: 0;
   display: flex;
   align-items: center;
   justify-content: center;