Comments

src/components/ha-top-app-bar-fixed.ts

@@ -2,13 +2,26 @@ import { TopAppBarFixedBase } from "@material/mwc-top-app-bar-fixed/mwc-top-app-
 import { styles } from "@material/mwc-top-app-bar/mwc-top-app-bar.css";
 import { css } from "lit";
 import { customElement, property } from "lit/decorators";
+import { ViewTransitionMixin } from "../mixins/view-transition-mixin";
+import { haStyleViewTransitions } from "../resources/styles";
 
 @customElement("ha-top-app-bar-fixed")
-export class HaTopAppBarFixed extends TopAppBarFixedBase {
+export class HaTopAppBarFixed extends ViewTransitionMixin(TopAppBarFixedBase) {
   @property({ type: Boolean, reflect: true }) public narrow = false;
 
+  @property({ type: Boolean, reflect: true, attribute: "content-loading" })
+  public contentLoading = true;
+
+  protected onLoadTransition(): void {
+    // Trigger the transition when content is slotted
+    this.startViewTransition(() => {
+      this.contentLoading = false;
+    });
+  }
+
   static override styles = [
     styles,
+    haStyleViewTransitions,
     css`
       header {
         padding-top: var(--safe-area-inset-top);
@@ -23,6 +36,10 @@ export class HaTopAppBarFixed extends TopAppBarFixedBase {
         );
         padding-bottom: var(--safe-area-inset-bottom);
         padding-right: var(--safe-area-inset-right);
+        view-transition-name: layout-fade-in;
+      }
+      :host([content-loading]) .mdc-top-app-bar--fixed-adjust {
+        opacity: 0;
       }
       :host([narrow]) .mdc-top-app-bar--fixed-adjust {
         padding-left: var(--safe-area-inset-left);

src/components/ha-two-pane-top-app-bar-fixed.ts

@@ -7,17 +7,18 @@ import type { MDCTopAppBarAdapter } from "@material/top-app-bar/adapter";
 import { strings } from "@material/top-app-bar/constants";
 import MDCFixedTopAppBarFoundation from "@material/top-app-bar/fixed/foundation";
 import { html, css, nothing } from "lit";
-import { property, query, customElement } from "lit/decorators";
+import { property, query, customElement, state } from "lit/decorators";
 import { classMap } from "lit/directives/class-map";
 import { styles } from "@material/mwc-top-app-bar/mwc-top-app-bar.css";
-import { haStyleScrollbar } from "../resources/styles";
+import { ViewTransitionMixin } from "../mixins/view-transition-mixin";
+import { haStyleScrollbar, haStyleViewTransitions } from "../resources/styles";
 
 export const passiveEventOptionsIfSupported = supportsPassiveEventListener
   ? { passive: true }
   : undefined;
 
 @customElement("ha-two-pane-top-app-bar-fixed")
-export class TopAppBarBaseBase extends BaseElement {
+export class TopAppBarBaseBase extends ViewTransitionMixin(BaseElement) {
   protected override mdcFoundation!: MDCFixedTopAppBarFoundation;
 
   protected override mdcFoundationClass = MDCFixedTopAppBarFoundation;
@@ -48,6 +49,15 @@ export class TopAppBarBaseBase extends BaseElement {
 
   @query(".pane .ha-scrollbar") private _paneElement?: HTMLElement;
 
+  @state() private _loaded = false;
+
+  protected onLoadTransition(): void {
+    // Trigger the transition when content is slotted
+    this.startViewTransition(() => {
+      this._loaded = true;
+    });
+  }
+
   @property({ attribute: false, type: Object })
   get scrollTarget() {
     return this._scrollTarget || window;
@@ -144,7 +154,12 @@ export class TopAppBarBaseBase extends BaseElement {
           : nothing}
         <div class="main">
           ${this.pane ? html`<div class="shadow-container"></div>` : nothing}
-          <div class="content">
+          <div
+            class=${classMap({
+              content: true,
+              loading: !this._loaded,
+            })}
+          >
             <slot></slot>
           </div>
         </div>
@@ -245,6 +260,7 @@ export class TopAppBarBaseBase extends BaseElement {
   static override styles = [
     styles,
     haStyleScrollbar,
+    haStyleViewTransitions,
     css`
       header {
         padding-top: var(--safe-area-inset-top);
@@ -341,6 +357,10 @@ export class TopAppBarBaseBase extends BaseElement {
       .mdc-top-app-bar--pane .content {
         height: 100%;
         overflow: auto;
+        view-transition-name: layout-fade-in;
+      }
+      .content.loading {
+        opacity: 0;
       }
       .mdc-top-app-bar__title {
         font-size: var(--ha-font-size-xl);

src/html/index.html.template

@@ -37,6 +37,7 @@
         flex-direction: column;
         justify-content: center;
         align-items: center;
+        view-transition-name: layout-fade-out;
       }
       #ha-launch-screen svg {
         width: 112px;

src/layouts/hass-loading-screen.ts

@@ -61,6 +61,7 @@ class HassLoadingScreen extends LitElement {
           display: block;
           height: 100%;
           background-color: var(--primary-background-color);
+          view-transition-name: layout-fade-out;
         }
         .toolbar {
           display: flex;

src/layouts/hass-router-page.ts

@@ -3,6 +3,7 @@ import { ReactiveElement } from "lit";
 import { property } from "lit/decorators";
 import memoizeOne from "memoize-one";
 import { navigate } from "../common/navigate";
+import { ViewTransitionMixin } from "../mixins/view-transition-mixin";
 import type { Route } from "../types";
 
 const extractPage = (path: string, defaultPage: string) => {
@@ -43,7 +44,7 @@ export interface RouterOptions {
 // Time to wait for code to load before we show loading screen.
 const LOADING_SCREEN_THRESHOLD = 400; // ms
 
-export class HassRouterPage extends ReactiveElement {
+export class HassRouterPage extends ViewTransitionMixin(ReactiveElement) {
   @property({ attribute: false }) public route?: Route;
 
   protected routerOptions!: RouterOptions;
@@ -310,16 +311,19 @@ export class HassRouterPage extends ReactiveElement {
     page: string,
     routeOptions: RouteOptions
   ) {
+    this.startViewTransition(() => {
       if (this.lastChild) {
         this.removeChild(this.lastChild);
       }
 
       const panelEl = this._cache[page] || this.createElement(routeOptions.tag);
+      (panelEl as HTMLElement).style.viewTransitionName = "layout-fade-in";
       this.updatePageEl(panelEl);
       this.appendChild(panelEl);
 
       if (routerOptions.cacheAll || routeOptions.cache) {
         this._cache[page] = panelEl;
       }
+    });
   }
 }

src/layouts/hass-subpage.ts

@@ -1,15 +1,17 @@
 import type { CSSResultGroup, TemplateResult } from "lit";
 import { css, html, LitElement } from "lit";
-import { customElement, eventOptions, property } from "lit/decorators";
+import { customElement, eventOptions, property, state } from "lit/decorators";
+import { classMap } from "lit/directives/class-map";
 import { restoreScroll } from "../common/decorators/restore-scroll";
 import { goBack } from "../common/navigate";
 import "../components/ha-icon-button-arrow-prev";
 import "../components/ha-menu-button";
-import { haStyleScrollbar } from "../resources/styles";
+import { ViewTransitionMixin } from "../mixins/view-transition-mixin";
+import { haStyleScrollbar, haStyleViewTransitions } from "../resources/styles";
 import type { HomeAssistant } from "../types";
 
 @customElement("hass-subpage")
-class HassSubpage extends LitElement {
+class HassSubpage extends ViewTransitionMixin(LitElement) {
   @property({ attribute: false }) public hass!: HomeAssistant;
 
   @property() public header?: string;
@@ -24,9 +26,18 @@ class HassSubpage extends LitElement {
 
   @property({ type: Boolean }) public supervisor = false;
 
+  @state() private _loaded = false;
+
   // @ts-ignore
   @restoreScroll(".content") private _savedScrollPos?: number;
 
+  protected onLoadTransition(): void {
+    // Trigger the transition when content is slotted
+    this.startViewTransition(() => {
+      this._loaded = true;
+    });
+  }
+
   protected render(): TemplateResult {
     return html`
       <div class="toolbar">
@@ -60,7 +71,14 @@ class HassSubpage extends LitElement {
           <slot name="toolbar-icon"></slot>
         </div>
       </div>
-      <div class="content ha-scrollbar" @scroll=${this._saveScrollPos}>
+      <div
+        class=${classMap({
+          content: true,
+          "ha-scrollbar": true,
+          loading: !this._loaded,
+        })}
+        @scroll=${this._saveScrollPos}
+      >
         <slot></slot>
       </div>
       <div id="fab">
@@ -85,6 +103,7 @@ class HassSubpage extends LitElement {
   static get styles(): CSSResultGroup {
     return [
       haStyleScrollbar,
+      haStyleViewTransitions,
       css`
         :host {
           display: block;
@@ -167,6 +186,10 @@ class HassSubpage extends LitElement {
           overflow-y: auto;
           overflow: auto;
           -webkit-overflow-scrolling: touch;
+          view-transition-name: layout-fade-in;
+        }
+        .content.loading {
+          opacity: 0;
         }
         :host([narrow]) .content {
           width: calc(

src/layouts/hass-tabs-subpage.ts

@@ -11,7 +11,8 @@ import "../components/ha-icon-button-arrow-prev";
 import "../components/ha-menu-button";
 import "../components/ha-svg-icon";
 import "../components/ha-tab";
-import { haStyleScrollbar } from "../resources/styles";
+import { ViewTransitionMixin } from "../mixins/view-transition-mixin";
+import { haStyleScrollbar, haStyleViewTransitions } from "../resources/styles";
 import type { HomeAssistant, Route } from "../types";
 
 export interface PageNavigation {
@@ -29,7 +30,7 @@ export interface PageNavigation {
 }
 
 @customElement("hass-tabs-subpage")
-class HassTabsSubpage extends LitElement {
+class HassTabsSubpage extends ViewTransitionMixin(LitElement) {
   @property({ attribute: false }) public hass!: HomeAssistant;
 
   @property({ type: Boolean }) public supervisor = false;
@@ -61,9 +62,18 @@ class HassTabsSubpage extends LitElement {
 
   @state() private _activeTab?: PageNavigation;
 
+  @state() private _loaded = false;
+
   // @ts-ignore
   @restoreScroll(".content") private _savedScrollPos?: number;
 
+  protected onLoadTransition(): void {
+    // Trigger the transition when content is slotted
+    this.startViewTransition(() => {
+      this._loaded = true;
+    });
+  }
+
   private _getTabs = memoizeOne(
     (
       tabs: PageNavigation[],
@@ -185,7 +195,12 @@ class HassTabsSubpage extends LitElement {
             </div>`
           : nothing}
         <div
-          class="content ha-scrollbar ${classMap({ tabs: showTabs })}"
+          class=${classMap({
+            content: true,
+            "ha-scrollbar": true,
+            tabs: showTabs,
+            loading: !this._loaded,
+          })}
           @scroll=${this._saveScrollPos}
         >
           <slot></slot>
@@ -214,6 +229,7 @@ class HassTabsSubpage extends LitElement {
   static get styles(): CSSResultGroup {
     return [
       haStyleScrollbar,
+      haStyleViewTransitions,
       css`
         :host {
           display: block;
@@ -332,6 +348,10 @@ class HassTabsSubpage extends LitElement {
           margin-bottom: var(--safe-area-inset-bottom);
           overflow: auto;
           -webkit-overflow-scrolling: touch;
+          view-transition-name: layout-fade-in;
+        }
+        .content.loading {
+          opacity: 0;
         }
         :host([narrow]) .content {
           margin-left: var(--safe-area-inset-left);

src/mixins/view-transition-mixin.ts

@@ -0,0 +1,94 @@
+import type { PropertyValues, ReactiveElement } from "lit";
+
+type AbstractConstructor<T = object> = abstract new (...args: any[]) => T;
+
+export const ViewTransitionMixin = <
+  T extends AbstractConstructor<ReactiveElement>,
+>(
+  superClass: T
+) => {
+  abstract class ViewTransitionClass extends superClass {
+    private _slot?: HTMLSlotElement;
+
+    private _transitionTriggered = false;
+
+    /**
+     * Trigger a view transition if supported by the browser
+     * @param updateCallback - Callback function that updates the DOM
+     * @returns Promise that resolves when the transition is complete
+     */
+    protected async startViewTransition(
+      updateCallback: () => void | Promise<void>
+    ): Promise<void> {
+      if (
+        !document.startViewTransition ||
+        window.matchMedia("(prefers-reduced-motion: reduce)").matches
+      ) {
+        // Fallback: update without view transition
+        await updateCallback();
+        return;
+      }
+
+      const transition = document.startViewTransition(async () => {
+        await updateCallback();
+      });
+
+      try {
+        await transition.finished;
+      } catch (_error) {
+        // View transition skipped
+      }
+    }
+
+    /**
+     * Optional callback to execute during the load transition
+     */
+    protected onLoadTransition?(): void;
+
+    /**
+     * Check if slot has content and trigger transition if it does
+     */
+    private _checkSlotContent = (): void => {
+      // Guard against multiple slotchange events triggering the transition multiple times
+      if (this._transitionTriggered) {
+        return;
+      }
+
+      if (this._slot) {
+        const elements = this._slot.assignedElements();
+        if (elements.length > 0) {
+          this._transitionTriggered = true;
+          this.onLoadTransition?.();
+        }
+      }
+    };
+
+    /**
+     * Automatically apply view transition on first render
+     * @param changedProperties - Properties that changed
+     */
+    protected firstUpdated(changedProperties: PropertyValues): void {
+      super.firstUpdated(changedProperties);
+
+      // Wait for slotted content to be ready, then trigger transition
+      this._slot = this.shadowRoot?.querySelector(
+        "slot:not([name])"
+      ) as HTMLSlotElement | undefined;
+      if (this._slot) {
+        this._checkSlotContent();
+        this._slot.addEventListener("slotchange", this._checkSlotContent);
+      } else {
+        // Start transition immediately if no slot is found
+        this.onLoadTransition?.();
+      }
+    }
+
+    override disconnectedCallback(): void {
+      super.disconnectedCallback();
+      if (this._slot) {
+        this._slot.removeEventListener("slotchange", this._checkSlotContent);
+      }
+    }
+  }
+  return ViewTransitionClass;
+};

src/panels/lovelace/hui-root.ts

@@ -72,7 +72,8 @@ import {
 } from "../../dialogs/quick-bar/show-dialog-quick-bar";
 import { showShortcutsDialog } from "../../dialogs/shortcuts/show-shortcuts-dialog";
 import { showVoiceCommandDialog } from "../../dialogs/voice-command-dialog/show-ha-voice-command-dialog";
-import { haStyle } from "../../resources/styles";
+import { ViewTransitionMixin } from "../../mixins/view-transition-mixin";
+import { haStyle, haStyleViewTransitions } from "../../resources/styles";
 import type { HomeAssistant, PanelInfo } from "../../types";
 import { documentationUrl } from "../../util/documentation-url";
 import { showToast } from "../../util/toast";
@@ -114,7 +115,7 @@ interface SubActionItem {
 }
 
 @customElement("hui-root")
-class HUIRoot extends LitElement {
+class HUIRoot extends ViewTransitionMixin(LitElement) {
   @property({ attribute: false }) public panel?: PanelInfo<LovelacePanelConfig>;
 
   @property({ attribute: false }) public hass!: HomeAssistant;
@@ -130,6 +131,8 @@ class HUIRoot extends LitElement {
 
   @state() private _curView?: number | "hass-unused-entities";
 
+  @state() private _loaded = false;
+
   private _viewCache?: Record<string, HUIView>;
 
   private _viewScrollPositions: Record<string, number> = {};
@@ -153,6 +156,10 @@ class HUIRoot extends LitElement {
     );
   }
 
+  protected onLoadTransition(): void {
+    this._loaded = true;
+  }
+
   private _renderActionItems(): TemplateResult {
     const result: TemplateResult[] = [];
     if (this._editMode) {
@@ -493,6 +500,7 @@ class HUIRoot extends LitElement {
         class=${classMap({
           "edit-mode": this._editMode,
           narrow: this.narrow,
+          loading: !this._loaded,
         })}
       >
         <div class="header">
@@ -1162,6 +1170,7 @@ class HUIRoot extends LitElement {
     // Recreate a new element to clear the applied themes.
     const root = this._viewRoot;
 
+    this.startViewTransition(() => {
       if (root.lastChild) {
         root.removeChild(root.lastChild);
       }
@@ -1199,6 +1208,7 @@ class HUIRoot extends LitElement {
       view.narrow = this.narrow;
 
       root.appendChild(view);
+    });
   }
 
   private _openShortcutDialog(ev: Event) {
@@ -1209,12 +1219,21 @@ class HUIRoot extends LitElement {
   static get styles(): CSSResultGroup {
     return [
       haStyle,
+      haStyleViewTransitions,
       css`
         :host {
           -ms-user-select: none;
           -webkit-user-select: none;
           -moz-user-select: none;
         }
+
+        @media (prefers-reduced-motion: no-preference) {
+          ::view-transition-new(hui-root-container) {
+            animation: fade-in var(--ha-animation-layout-duration) ease-out;
+            animation-delay: var(--ha-animation-layout-delay-base);
+          }
+        }
+
         .header {
           background-color: var(--app-header-background-color);
           color: var(--app-header-text-color, white);
@@ -1403,6 +1422,10 @@ class HUIRoot extends LitElement {
           padding-right: var(--safe-area-inset-right);
           padding-inline-end: var(--safe-area-inset-right);
           padding-bottom: var(--safe-area-inset-bottom);
+          view-transition-name: hui-root-container;
+        }
+        .loading hui-view-container {
+          opacity: 0;
         }
         .narrow hui-view-container {
           padding-left: var(--safe-area-inset-left);
@@ -1411,6 +1434,7 @@ class HUIRoot extends LitElement {
         hui-view-container > * {
           flex: 1 1 100%;
           max-width: 100%;
+          view-transition-name: layout-fade-in;
         }
         /**
          * In edit mode we have the tab bar on a new line *

src/resources/styles.ts

@@ -199,3 +199,56 @@ export const baseEntrypointStyles = css`
     width: 100vw;
   }
 `;
+
+export const haStyleViewTransitions = css`
+  @keyframes fade-in {
+    from {
+      opacity: 0;
+    }
+    to {
+      opacity: 1;
+    }
+  }
+
+  @keyframes fade-out {
+    from {
+      opacity: 1;
+    }
+    to {
+      opacity: 0;
+    }
+  }
+
+  @media (prefers-reduced-motion: no-preference) {
+    /* Prevent root cross-fade during view transitions (pseudo-element) */
+    ::view-transition-old(root) {
+      animation: none;
+    }
+    ::view-transition-new(root) {
+      animation: none;
+    }
+
+    /* Elements leaving the view (loading screen) */
+    ::view-transition-group(layout-fade-out) {
+      animation-duration: var(--ha-animation-layout-duration);
+      animation-timing-function: ease-out;
+    }
+    ::view-transition-old(layout-fade-out) {
+      animation: fade-out var(--ha-animation-layout-duration) ease-out;
+    }
+    ::view-transition-new(layout-fade-out) {
+      animation: none;
+    }
+
+    /* New content entering (panels, subpages)
+       Uses base delay to be less abrupt and allow for elements to render */
+    ::view-transition-group(layout-fade-in) {
+      animation-duration: var(--ha-animation-layout-duration);
+      animation-timing-function: ease-out;
+    }
+    ::view-transition-new(layout-fade-in) {
+      animation: fade-in var(--ha-animation-layout-duration) ease-out;
+      animation-delay: var(--ha-animation-layout-delay-base);
+    }
+  }
+`;

src/resources/theme/core.globals.ts

@@ -42,6 +42,17 @@ export const coreStyles = css`
     --ha-space-18: 72px;
     --ha-space-19: 76px;
     --ha-space-20: 80px;
+
+    /* Animation timing */
+    --ha-animation-layout-duration: 350ms;
+    --ha-animation-layout-delay-base: 100ms;
+  }
+
+  @media (prefers-reduced-motion: reduce) {
+    html {
+      --ha-animation-layout-duration: 0ms;
+      --ha-animation-layout-delay-base: 0ms;
+    }
   }
 `;
 

src/util/launch-screen.ts

@@ -3,7 +3,20 @@ import { render } from "lit";
 
 export const removeLaunchScreen = () => {
   const launchScreenElement = document.getElementById("ha-launch-screen");
-  if (launchScreenElement) {
+  if (!launchScreenElement) {
+    return;
+  }
+
+  // Use View Transition API if available and user doesn't prefer reduced motion
+  if (
+    document.startViewTransition &&
+    !window.matchMedia("(prefers-reduced-motion: reduce)").matches
+  ) {
+    document.startViewTransition(() => {
+      launchScreenElement.parentElement!.removeChild(launchScreenElement);
+    });
+  } else {
+    // Fallback: Direct removal without transition
     launchScreenElement.parentElement!.removeChild(launchScreenElement);
   }
 };