feat(player): drum-part picker for multiple drum charts (feedpak 1.17.0)

The last mile of the multiple-drum-parts feature: let a player CHOOSE which
drum chart plays. #1020 taught the loader + highway WS to carry several drum
parts (song_info.drum_parts + ?drum_part=<id> + a part_id echo on drum_tab);
this adds the host-chrome selector that drives it.

A "Drum part" <select> sits beside the arrangement switcher in the advanced
settings popover, shown only when a song has 2+ drum charts (drum_parts is
always present — empty for non-drum songs — so single-drum / no-drum songs
hide the row and nothing changes for them). Selecting a part re-streams that
part's tab over the highway WS, exactly like an arrangement switch.

- static/highway.js:
  - reconnect() gains a third `drumPart` arg → sets `?drum_part=<id>` on the WS
    URL (mirrors the existing `arrangement` param one line up). Empty/undefined
    → the primary part, i.e. byte-identical to today for any pack untouched.
  - song_info handler populates #drum-part-select from msg.drum_parts and
    shows/hides #v3-drum-part-row on `length > 1` (parallel to the #arr-select
    block right above it).
  - drum_tab handler carries msg.part_id onto hwState.drumTab (plugins can read
    bundle.drumTab.part_id) and reflects it as the picker's selected value, so
    the dropdown stays honest even when the server resolves an unknown/absent
    selection to the primary.
- static/app.js:
  - changeArrangement() gains an optional `drumPart`; at reconnect it forwards
    the explicit part, else preserves the current picker selection — so an
    ARRANGEMENT switch keeps the chosen drum part (parts are song-level).
  - new changeDrumPart(id) delegates to changeArrangement with the current
    arrangement held + the new part applied (a part switch is the same
    re-stream, so it reuses all the transition ceremony). Exported on window.
- static/v3/index.html: the #drum-part-select row (hidden by default).

No plugin change: the drum renderers just draw whatever drum_tab streams.

RUNTIME-VERIFIED (Playwright, the core player, a 2-drum pack + a no-drum pack):
10/10 — the picker populates with both parts and shows for the multi-drum song;
song_info.drum_parts reaches getSongInfo(); the primary is pre-selected;
selecting the 2nd part drives highway.reconnect with the id and the WS URL
carries `?drum_part=drums-2`; the picker then reflects the server's part_id
echo; a no-drum song hides the row; no page errors. ESLint 0 errors (the two
max-lines warnings are pre-existing on these files). No pytest touched (JS-only).
Stacked on #1020.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xGPjDBF8NTwTK7VQvizix
Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

CHANGELOG.md

@@ -8,6 +8,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Drum-part picker (feedpak 1.17.0 "drums as arrangements").** When a song
+  carries several drum charts, a **Drum part** selector appears beside the
+  arrangement switcher (advanced settings) so a player can choose which drummer
+  to play. Selecting one re-streams that part's tab over the highway WS
+  (`?drum_part=<id>`, mirroring the arrangement switch); the choice persists
+  across an arrangement change, and the picker reflects the server's
+  authoritative part (unknown/absent selection falls back to the primary). The
+  row hides for single-drum and non-drum songs, so nothing changes there. Builds
+  on the loader below; no plugin change needed — the drum renderer just draws
+  whatever tab streams.
 - **Multiple drum parts (feedpak 1.17.0 "drums as arrangements").** The sloppak
   loader now reads `type: drums` arrangement entries carrying per-arrangement
   `drum_tab` file pointers — a song can ship several drum charts (a second

static/app.js

@@ -1150,7 +1150,7 @@ window.feedBack.on('song:ready', () => {
 let _arrBusyGen = 0;
 let _arrBusyTimeout = null;
 
-async function changeArrangement(index) {
+async function changeArrangement(index, drumPart) {
     if (currentFilename) {
         // Tear down any pending fresh-load credits before switching: the
         // no-count-in hold timer would otherwise fire togglePlay() against the
@@ -1276,11 +1276,38 @@ async function changeArrangement(index) {
         _resetSectionPracticeLog();
         invalidateParentCount();
 
-        window.highway.reconnect(currentFilename, index);
+        // Carry the selected drum part across the re-stream. An explicit
+        // `drumPart` (a drum-part switch, from changeDrumPart) wins; otherwise
+        // preserve the current picker selection so an ARRANGEMENT switch keeps
+        // the chosen part (drum parts are song-level, not per-arrangement).
+        const part = drumPart !== undefined
+            ? drumPart
+            : (document.getElementById('drum-part-select')?.value || '');
+        window.highway.reconnect(currentFilename, index, part);
         window.feedBack.emit('arrangement:changed', { index, filename: currentFilename });
     }
 }
 
+// Switch which drum part plays (feedpak 1.17.0 "drums as arrangements"). A part
+// switch re-streams the same song with a different drum tab — the same
+// transition as an arrangement switch — so it delegates to changeArrangement
+// with the CURRENT arrangement held and the new part applied. Wired to
+// #drum-part-select's onchange; the select is populated + shown by
+// highway.js's song_info handler only when the song has 2+ drum parts.
+async function changeDrumPart(partId) {
+    if (!currentFilename) return;
+    let index = 0;
+    const si = window.highway && typeof window.highway.getSongInfo === 'function'
+        ? window.highway.getSongInfo() : null;
+    if (si && typeof si.arrangement_index === 'number' && si.arrangement_index >= 0) {
+        index = si.arrangement_index;
+    } else {
+        const arrSel = document.getElementById('arr-select');
+        if (arrSel && arrSel.value !== '') index = Number(arrSel.value) || 0;
+    }
+    return changeArrangement(index, partId);
+}
+
 // Restart the current song from the beginning (or from loop A when an A–B
 // loop is armed). Uses the canonical _audioSeek funnel only — never touches
 // audio.currentTime directly and never reloads via playSong().
@@ -2325,7 +2352,7 @@ configureHost({
 Object.assign(window, {
     _confirmDialog, _getArrangementNamingMode, _libraryLocalFilename, _librarySongArtUrl,
     _librarySongId, _onHeaderClick, _onNamingModeChange, _trapFocusInModal,
-    changeArrangement, checkPluginUpdates, clearLibFilters, clearLoop,
+    changeArrangement, changeDrumPart, checkPluginUpdates, clearLibFilters, clearLoop,
     deleteSelectedLoop, esc, exportDiagnostics, exportSettings, filterFavorites,
     filterLibrary, fullRescanLibrary, goFavPage, handleSliderInput,
     hideScanBanner, importSettings, loadPlugins, loadSavedLoop,

static/highway.js

@@ -2298,6 +2298,31 @@ function createHighway() {
                                         sel.appendChild(opt);
                                     }
                                 }
+                                // Drum-part picker (feedpak 1.17.0 "drums as
+                                // arrangements"): a song can carry several drum
+                                // charts. Populate the picker beside the
+                                // arrangement switcher; show it only when there
+                                // are 2+ parts to choose between. `drum_parts`
+                                // is always present (empty for non-drum songs),
+                                // so a single-drum / no-drum song hides it. The
+                                // currently-streaming part is marked selected by
+                                // the `drum_tab` handler below (authoritative
+                                // `part_id`), so we don't guess here.
+                                {
+                                    const dpSel = document.getElementById('drum-part-select');
+                                    if (dpSel) {
+                                        const parts = Array.isArray(msg.drum_parts) ? msg.drum_parts : [];
+                                        dpSel.textContent = '';
+                                        for (const p of parts) {
+                                            const opt = document.createElement('option');
+                                            opt.value = p.id;
+                                            opt.textContent = p.name || p.id;
+                                            dpSel.appendChild(opt);
+                                        }
+                                        const dpRow = document.getElementById('v3-drum-part-row');
+                                        if (dpRow) dpRow.classList.toggle('hidden', parts.length <= 1);
+                                    }
+                                }
                             }
                             // Plugin context API — broadcast current song state
                             if (window.feedBack) {
@@ -2380,7 +2405,22 @@ function createHighway() {
                                 name: (typeof msg.name === 'string' && msg.name) ? msg.name : 'Drums',
                                 kit: Array.isArray(msg.kit) ? msg.kit : [],
                                 hits: [],
+                                // Which drum part this stream carries (feedpak
+                                // 1.17.0). Present only for multi-part packs;
+                                // null otherwise. Plugins can read it via
+                                // bundle.drumTab.part_id.
+                                part_id: (typeof msg.part_id === 'string' && msg.part_id) ? msg.part_id : null,
                             };
+                            // Reflect the authoritative streaming part in the
+                            // picker (the server resolves an unknown/absent
+                            // selection to the primary, so this keeps the
+                            // dropdown honest even after a fallback).
+                            if (hwState.drumTab.part_id) {
+                                const dpSel = document.getElementById('drum-part-select');
+                                if (dpSel && dpSel.value !== hwState.drumTab.part_id) {
+                                    dpSel.value = hwState.drumTab.part_id;
+                                }
+                            }
                             break;
                         case 'drum_hits':
                             if (hwState.drumTab && Array.isArray(msg.data)) {
@@ -2773,7 +2813,7 @@ function createHighway() {
             localStorage.setItem('showFingerHints', String(hwState._showFingerHints));
         },
 
-        reconnect(filename, arrangement) {
+        reconnect(filename, arrangement, drumPart) {
             // Close old WS but keep audio + animation running
             if (hwState.ws) { hwState.ws.close(); hwState.ws = null; }
             hwState.ready = false;
@@ -2799,6 +2839,11 @@ function createHighway() {
             _resetChordRenderState();
             const wsParams = new URLSearchParams();
             if (arrangement !== undefined) wsParams.set('arrangement', arrangement);
+            // Multiple drum parts (feedpak 1.17.0 "drums as arrangements"):
+            // carry the selected part id so the WS streams ITS drum tab. Empty
+            // / undefined → the primary part (server default), i.e. today's
+            // one-drum behavior for any pack the picker never touched.
+            if (drumPart) wsParams.set('drum_part', drumPart);
             let namingMode = 'smart';
             if (typeof window._getArrangementNamingMode === 'function') {
                 const v = window._getArrangementNamingMode();

static/v3/index.html

@@ -1194,6 +1194,10 @@
                         <button id="arr-default-pin" type="button" onclick="pinCurrentArrangementDefault()" aria-pressed="false" aria-label="Select an arrangement to make it the default" class="v3-pop-pin" title="Select an arrangement to make it the default">☆</button>
                     </span>
                 </div>
+                <div class="v3-pop-row hidden" id="v3-drum-part-row">
+                    <span class="v3-pop-label">Drum part</span>
+                    <select id="drum-part-select" onchange="changeDrumPart(this.value)" class="v3-pop-select max-w-[130px]" title="Which drum chart to play — a song can carry several"></select>
+                </div>
                 <div class="v3-pop-row">
                     <span class="v3-pop-label" id="mastery-slider-label">Difficulty</span>
                     <span class="flex items-center gap-2">