Animate room rank changes

static/app.js

@@ -8,6 +8,8 @@ const BASE = document.documentElement.dataset.base || '/';
 const POLL_MS = 5000;
 // How long a dropped cursor waits for the player to report it got there.
 const SEEK_HOLD_MS = 10000;
+// Motion turned down in the OS means cards change places without sliding.
+const CALM = matchMedia('(prefers-reduced-motion: reduce)');
 
 const el = (sel, root = document) => root.querySelector(sel);
 const els = (sel, root = document) => Array.from(root.querySelectorAll(sel));
@@ -58,6 +60,7 @@ const ui = {
   inputIcon: el('[data-role="input-icon"]'),
   sheet: el('[data-role="sheet"]'),
   options: el('[data-role="options"]'),
+  app: el('.app'),
   avr: el('.card.avr'),
   joined: el('[data-role="joined"]'),
   avrMedia: el('[data-role="avr-media"]'),
@@ -91,6 +94,10 @@ const avrTrack = {
 avrTrack.cover.addEventListener('error', () => { avrTrack.cover.hidden = true; });
 seekable(avrTrack);
 
+// Same as a room's card: named, so a view transition can follow it when it
+// moves to the top and back.
+ui.avr.style.viewTransitionName = 'avr';
+
 const rooms = {};
 let inputs = [];
 let currentInput = null;
@@ -124,7 +131,6 @@ els('.room').forEach((node) => {
   const room = {
     key,
     node,
-    slot: el(`.room-slot[data-slot="${key}"]`),
     level: el('[data-role="level"]', node),
     meter: el('[data-role="meter"]', node),
     actions: el('[data-role="actions"]', node),
@@ -225,9 +231,8 @@ function paintRoom(room) {
   // Out of the group the button names where tapping it sends the room; in it,
   // where the card sits already says so, so the button says what it does.
   room.toggleLabel.textContent = room.grouped ? 'Ungroup' : 'Cinema';
-  // Grouped, its slot is empty and the AVR's card is the one that moves up.
-  room.slot.classList.toggle('on-top', playing && !room.grouped);
   placeRoom(room);
+  placeCards();
 }
 
 /* Song, artist, cover and progress, each only when there is one. The
@@ -427,13 +432,24 @@ function clock(seconds) {
 }
 
 /* A merged room moves into the host's card, because that is what merging
-   means: one group, playing one thing. Leaving puts the card back in its
-   own slot, which is why the slots exist. */
+   means: one group, playing one thing. Leaving puts it back in the column,
+   ahead of the rooms declared after it, so the cards keep the order they are
+   declared in however they were grouped. */
 function placeRoom(room) {
-  const target = room.grouped ? ui.joined : room.slot;
-  if (room.node.parentElement !== target) target.appendChild(room.node);
-  // Left empty, a slot would still take a gap of its own and double the space.
-  room.slot.hidden = room.grouped;
+  if (room.grouped) {
+    if (room.node.parentElement !== ui.joined) ui.joined.appendChild(room.node);
+  } else if (room.node.parentElement !== ui.app) {
+    const order = Object.keys(rooms);
+    const next = order.slice(order.indexOf(room.key) + 1)
+      .map((key) => rooms[key].node)
+      .find((node) => node.parentElement === ui.app);
+    // Nothing left in the column below it means it goes last, before the host.
+    ui.app.insertBefore(room.node, next || ui.avr);
+  }
+  // A view transition only follows an element it can name, and the name has
+  // to be one of a kind on the page (see placeCards). Inside the host's card
+  // the card travels with it, so it takes no name of its own there.
+  room.node.style.viewTransitionName = room.grouped ? 'none' : `room-${room.key}`;
 }
 
 function paintGrouping() {
@@ -451,7 +467,36 @@ function paintGrouping() {
   const playing = inside.some((node) => node.classList.contains('playing'))
     || avrNowPlaying?.play_state === 'play';
   ui.avr.classList.toggle('playing', playing);
-  ui.avr.classList.toggle('on-top', playing);
+  placeCards();
+}
+
+/* Which card sits at the top of the column is CSS order (see .on-top), and
+   order is not a property that transitions: the cards change places between
+   one frame and the next. A view transition does the sliding instead -- the
+   browser snapshots the cards where they are, runs the swap, and animates
+   them to where they landed. One turn can paint several cards (a poll
+   touches every room and then the group), so the move is collected and run
+   once, and only started when a card really does change places. Without
+   support, or with motion turned down, the cards just change places the way
+   they always did. */
+let placing = false;
+function placeCards() {
+  if (placing) return;
+  placing = true;
+  queueMicrotask(() => {
+    placing = false;
+    // Grouped, a room's card has left the column and the AVR's is the one
+    // that moves up. The AVR's tint and its position answer the same
+    // question, and paintGrouping() has just settled it.
+    const wanted = Object.values(rooms)
+      .map((room) => [room.node, room.playing() && !room.grouped]);
+    wanted.push([ui.avr, ui.avr.classList.contains('playing')]);
+
+    const move = () => wanted.forEach(([node, top]) => node.classList.toggle('on-top', top));
+    const moves = wanted.some(([node, top]) => node.classList.contains('on-top') !== top);
+    if (!moves || CALM.matches || !document.startViewTransition) move();
+    else document.startViewTransition(move);
+  });
 }
 
 /* Press and hold to keep moving, accelerating as you hold. */

static/panel.css

@@ -89,7 +89,14 @@ svg { width: 22px; height: 22px; fill: currentColor; }
 
 /* Card */
 
-.app > .on-top { order: -1; } /* A playing card goes to the top */
+/* A playing card goes to the top. order does not transition, so the swap
+   rides on a view transition instead -- see placeCards() in app.js. */
+.app > .on-top { order: -1; }
+
+::view-transition-group(*) {
+  animation-duration: .4s;
+  animation-timing-function: cubic-bezier(.22, 1, .36, 1);
+}
 
 .card {
   background: var(--card);
@@ -321,8 +328,8 @@ button.sources-btn.loading .icon { display: none; }
 
 /* Footer */
 
-.foot { margin: var(--space-xs) var(--space-xs) 0; min-height: 18px; font-size: 12px; color: var(--muted); text-align: center; }
-.foot.bad { color: var(--warn); }
+footer { margin: var(--space-xs) var(--space-xs) 0; min-height: 18px; font-size: 12px; color: var(--muted); text-align: center; }
+footer.bad { color: var(--warn); }
 
 /* Toast */
 

templates/index.html

@@ -1,5 +1,6 @@
 <!doctype html>
 <html lang="en" data-step="{{ step }}" data-host="{{ host.label }}" data-base="{{ url_for('index') }}">
+
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover, user-scalable=no">
@@ -17,8 +18,8 @@
   <link rel="icon" href="{{ url_for('static', filename='logo.svg') }}">
   <link rel="stylesheet" href="{{ url_for('static', filename='panel.css') }}">
 </head>
-<body>
 
+<body>
 {% macro host_toggle() -%}
   <button class="sources-btn toggle" data-role="group" aria-pressed="false">
     <svg class="cinema icon" viewBox="0 0 512 512" aria-hidden="true"><path d="M256 512a256 256 0 1 0 0-512 256 256 0 1 0 0 512zM128 192c0-17.7 14.3-32 32-32l128 0c17.7 0 32 14.3 32 32l0 38.4 61-36.6c1.9-1.2 4.2-1.8 6.4-1.8 6.9 0 12.5 5.6 12.5 12.5l0 102.9c0 6.9-5.6 12.5-12.5 12.5-2.3 0-4.5-.6-6.4-1.8l-61-36.6 0 38.4c0 17.7-14.3 32-32 32l-128 0c-17.7 0-32-14.3-32-32l0-128z"/></svg>
@@ -52,7 +53,6 @@
   </header>
 
   {% for room in rooms %}
-  <div class="room-slot" data-slot="{{ room.key }}">
   <section class="card room" data-room="{{ room.key }}">
     <div class="card-head">
       <h2><svg class="room-icon" data-icon="{{ room.kind }}" viewBox="0 0 512 512" aria-hidden="true"><path d=""/></svg><span class="room-label">{{ room.label }}</span></h2>
@@ -105,7 +105,6 @@
 
     {{ control_row(room.key, spotify_accounts) }}
   </section>
-  </div>
   {% endfor %}
 
   <section class="card avr">
@@ -142,7 +141,7 @@
     <div class="joined" data-role="joined" hidden></div>
   </section>
 
-  <p class="foot" data-role="foot"></p>
+  <footer data-role="foot"></footer>
 </div>
 
 <div class="sheet-wrap" data-role="sheet" hidden>