refactor(app): lift the shared player state onto a container (R3a)

static/js/player-state.js

@@ -0,0 +1,34 @@
+// Shared, MUTABLE player state.
+//
+// WHY A CONTAINER AND NOT PLAIN EXPORTS. An imported binding is read-only. Every
+// slice carved out of app.js so far has only ever READ the state it shares
+// (loopA/loopB, _audioSeekGen, currentFilename), so a getter hook was enough and no
+// container was needed. That runs out here: count-in genuinely WRITES `isPlaying`
+// (it starts and stops playback) and `lastAudioTime`. `import { isPlaying }` then
+// `isPlaying = true` throws — the binding cannot be assigned to.
+//
+// So the state moves onto an object. `S.isPlaying = true` is a property write, which
+// works from any module holding the same `S`. This is the same shape the stems,
+// studio, and editor migrations converged on.
+//
+// It is deliberately SMALL. app.js has ~104 top-level `let` scalars; lifting all of
+// them would be a ~977-site rewrite for no benefit, since most are private to one
+// cluster and travel with it. Only the ones a carved module must WRITE belong here.
+// Add to it when a carve actually needs it, not before.
+//
+// NB app.js's own 71 reference sites were rewritten mechanically — but from the AST,
+// not by text substitution. Of 100 textual occurrences of these two names, only 71
+// resolve to the module binding: 22 are member accesses (`someObj.isPlaying`), 4 are
+// the local parameter of setPlayButtonState(isPlaying), one is an object key, and two
+// are shorthand properties (`{ isPlaying }`) that must become `{ isPlaying: S.isPlaying }`.
+// A blind find-and-replace corrupts all 29.
+export const S = {
+    /** Is the transport running? Written by playback, count-in, and the JUCE shims. */
+    isPlaying: false,
+
+    /**
+     * The last audio position we saw, in seconds. Used to detect a seek that did not
+     * land where it was asked to (JUCE can clamp; HTML5 can round).
+     */
+    lastAudioTime: 0,
+};

tests/js/juce_engine_reroute.test.js

@@ -84,7 +84,11 @@ function makeSandbox({ isAudioRunning, loadBackingTrack, outputType = 'Windows A
             json: () => Promise.resolve({ path: '/local/song.ogg' }),
         }),
         document: { hidden: false },
-        isPlaying: true,
+        // `isPlaying` moved onto the shared player-state container so a carved module
+        // can WRITE it (an imported binding is read-only). The sliced code now reads and
+        // writes S.isPlaying, so the sandbox provides the same container — the
+        // assertions below are unchanged.
+        S: { isPlaying: true, lastAudioTime: 0 },
         audio,
         jucePlayer,
         __calls: calls,

tests/js/loop_restart.test.js

@@ -55,8 +55,10 @@ function buildSandbox() {
         loopA: 10,
         loopB: 20,
         _countingIn: false,
-        isPlaying: false,
-        lastAudioTime: 0,
+        // isPlaying / lastAudioTime moved onto the shared player-state container
+        // (static/js/player-state.js) so a carved module can WRITE them — an imported
+        // binding is read-only. Same values, same assertions, one indirection.
+        S: { isPlaying: false, lastAudioTime: 0 },
 
         // Browser-ish globals.
         performance: { now: () => Date.now() },
@@ -135,8 +137,7 @@ test('loop:restart fires once when wrap path runs', async () => {
         var _countInGen = 0;
         var _countInTimer = null;
         var _countInRaf = 0;
-        var isPlaying = false;
-        var lastAudioTime = 0;
+        var S = { isPlaying: false, lastAudioTime: 0 };
         ${startCountInSrc}
         globalThis.__startCountIn = startCountIn;
     `;
@@ -180,8 +181,7 @@ test('loop:restart aborts when seek lands far from loopA (JUCE rollback)', async
         var _countInGen = 0;
         var _countInTimer = null;
         var _countInRaf = 0;
-        var isPlaying = false;
-        var lastAudioTime = 0;
+        var S = { isPlaying: false, lastAudioTime: 0 };
         ${startCountInSrc}
         globalThis.__startCountIn = startCountIn;
         globalThis.__getCountingIn = () => _countingIn;

tests/js/play_button_reroute_guard.test.js

@@ -29,8 +29,11 @@ async function runTogglePlayRejecting({ rerouteInProgress }) {
     const buttonStates = [];
     const sandbox = {
         console: { log() {}, warn() {}, error() {} },
-        // not-playing -> togglePlay takes the HTML5 play branch
-        isPlaying: false,
+        // not-playing -> togglePlay takes the HTML5 play branch.
+        // isPlaying / lastAudioTime moved onto the shared player-state container
+        // (static/js/player-state.js) so a carved module can WRITE them — an imported
+        // binding is read-only. Same values, same assertions, one indirection.
+        S: { isPlaying: false, lastAudioTime: 0 },
         _audioSeekGen: 0,
         _playAttemptGen: 0,
         setPlayButtonState(v) { buttonStates.push(v); },
@@ -51,7 +54,7 @@ async function runTogglePlayRejecting({ rerouteInProgress }) {
     vm.createContext(sandbox);
     vm.runInContext(TOGGLE_PLAY_SRC, sandbox, { filename: 'app.js#togglePlay' });
     await vm.runInContext('togglePlay()', sandbox);
-    return { buttonStates, isPlaying: sandbox.isPlaying };
+    return { buttonStates, isPlaying: sandbox.S.isPlaying };
 }
 
 test('reroute-aborted play() leaves the button on Pause (isPlaying stays true)', async () => {

tests/js/playback_app_adapter.test.js

@@ -84,5 +84,8 @@ test('playback adapter suppresses duplicate HTML5 pause events before emitting c
     const src = fs.readFileSync(APP_JS, 'utf8');
     const fn = extractFunction(src, 'function _installPlaybackTransportAdapter()');
 
-    assert.match(fn, /if \(!window\._juceMode && wasPlaying\) \{\s*isPlaying = false;\s*window\.feedBack\.isPlaying = false;\s*audio\.pause\(\);\s*_markPlaybackPaused\(\);\s*\}/);
+    // isPlaying moved onto the shared player-state container so a carved module can
+    // WRITE it (an imported binding is read-only). window.feedBack.isPlaying — the
+    // public mirror — is unchanged.
+    assert.match(fn, /if \(!window\._juceMode && wasPlaying\) \{\s*S\.isPlaying = false;\s*window\.feedBack\.isPlaying = false;\s*audio\.pause\(\);\s*_markPlaybackPaused\(\);\s*\}/);
 });

tests/js/song_restart.test.js

@@ -19,7 +19,9 @@ function buildSandbox({ loopA = null, loopB = null, isPlaying = false } = {}) {
     const sandbox = {
         loopA,
         loopB,
-        isPlaying,
+        // isPlaying moved onto the shared player-state container so a carved module can
+        // WRITE it (an imported binding is read-only). Same value, same assertions.
+        S: { isPlaying, lastAudioTime: 0 },
         __cancelCountInCalls: 0,
         __seekCalls: [],
         __startCountInCalls: [],
@@ -42,7 +44,7 @@ function buildSandbox({ loopA = null, loopB = null, isPlaying = false } = {}) {
         },
         __togglePlay() {
             sandbox.__togglePlayCalls++;
-            sandbox.isPlaying = true;
+            sandbox.S.isPlaying = true;
             return Promise.resolve();
         },
     };
@@ -53,7 +55,7 @@ function buildSandbox({ loopA = null, loopB = null, isPlaying = false } = {}) {
 function loadRestart(sandbox, src, { audioSeekImpl } = {}) {
     const restartSrc = extractFunction(src, 'async function restartCurrentSong(');
     const code = `
-        var isPlaying = ${sandbox.isPlaying};
+        var S = { isPlaying: ${sandbox.S.isPlaying}, lastAudioTime: 0 };
         function _cancelCountIn() { __cancelCountInCalls++; }
         async function _audioSeek(s, reason) {
             return (${audioSeekImpl || '__audioSeek'})(s, reason);

tests/js/song_seek.test.js

@@ -77,7 +77,10 @@ function loadFunctions(sandbox, src) {
         // _audioSeek now syncs the jump-fix tracker so far seeks don't
         // trigger an immediate revert; declare it here so the sandbox
         // assignment lands on a real binding rather than an implicit global.
-        let lastAudioTime = 0;
+        // lastAudioTime moved onto the shared player-state container
+        // (static/js/player-state.js) so a carved module can WRITE it — an imported
+        // binding is read-only. The sliced code writes S.lastAudioTime now.
+        let S = { isPlaying: false, lastAudioTime: 0 };
         // _audioSeek wraps jucePlayer.seek in a timeout race; pull in the
         // helper + constant. Tests can override jucePlayer.seek to vary
         // behavior; the timeout (2 s) is well above any test setTimeout.

tests/js/speed_reset.test.js

@@ -143,7 +143,10 @@ function loadPlaySong(sandbox) {
         : '';
     const code = `
         var artAbortController = null;
-        var isPlaying = true;
+        // isPlaying moved onto the shared player-state container so a carved module can
+        // WRITE it (an imported binding is read-only). NB window.feedBack.isPlaying — the
+        // public mirror stubbed above — is a different thing and is unchanged.
+        var S = { isPlaying: true, lastAudioTime: 0 };
         var currentFilename = null;
         var _playerOriginScreen = null;
         var _pendingAutostart = false;

tests/test_plugin_runtime_idempotence.py

@@ -186,7 +186,9 @@ def test_app_event_bus_dispatches_locally_and_preserves_juce_stop_state():
     source = (ROOT / "static" / "app.js").read_text(encoding="utf-8")
 
     assert "this.dispatchEvent(new CustomEvent(event, { detail }))" in source
-    assert "const hadPlayableSong = !!audio.src || !!window._juceAudioUrl || isPlaying" in source
+    # `isPlaying` moved onto the shared player-state container (static/js/player-state.js)
+    # so a carved module can WRITE it — an imported binding is read-only.
+    assert "const hadPlayableSong = !!audio.src || !!window._juceAudioUrl || S.isPlaying" in source
     assert "sm.emit('song:resume', payload)" in source
     assert "window.feedBack.emit('song:resume', payload)" in source