diff --git a/screen.js b/screen.js
index 3cd7952..4055811 100644
--- a/screen.js
+++ b/screen.js
@@ -2101,4 +2101,14 @@ window.slopsmithViz_drums = createFactory;
 // slopsmith→feedBack rename: host viz picker looks up `window.feedBackViz_<id>`.
 window.feedBackViz_drums = window.slopsmithViz_drums;
 
+// Node-only export hook for tests; browsers keep the window.*Viz_drums wiring.
+if (typeof module !== 'undefined' && module.exports) {
+    module.exports = {
+        noteToMidi, _rgbStr, _validateCustomMapping, _drumTabHitsToNotes,
+        _applyLanePreset, _getActiveDrumMap, _midiToLaneIdx, _songNoteToLaneIdx,
+        _midiResolveSaved, DRUM_LANES, PIECE_DEFAULT_MIDI,
+        matchesArrangement: createFactory.matchesArrangement,
+    };
+}
+
 })();
diff --git a/tests/screen.test.js b/tests/screen.test.js
new file mode 100644
index 0000000..54a025b
--- /dev/null
+++ b/tests/screen.test.js
@@ -0,0 +1,140 @@
+'use strict';
+// Coverage for pure helpers in screen.js: MIDI mapping, lane presets,
+// drum-tab hit conversion, custom-mapping validation, arrangement matching.
+// Runs under the org reusable CI as `node tests/screen.test.js`.
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const path = require('node:path');
+
+function freshPlugin() {
+    global.window = {};
+    global.localStorage = { getItem: () => null, setItem: () => {} };
+    global.document = { addEventListener: () => {} };
+    const file = path.join(__dirname, '..', 'screen.js');
+    delete require.cache[require.resolve(file)];
+    return require(file);
+}
+
+test('noteToMidi maps string/fret to a MIDI number (24 semitones per string)', () => {
+    const mod = freshPlugin();
+    assert.equal(mod.noteToMidi(0, 0), 0);
+    assert.equal(mod.noteToMidi(1, 12), 36);
+});
+
+test('_rgbStr formats rgb()/rgba() from 0..1 channel floats', () => {
+    const mod = freshPlugin();
+    assert.equal(mod._rgbStr(1, 0, 0), 'rgb(255,0,0)');
+    assert.equal(mod._rgbStr(1, 0, 0, 0.5), 'rgba(255,0,0,0.5)');
+});
+
+test('_validateCustomMapping keeps only in-range MIDI keys with known lane ids', () => {
+    const mod = freshPlugin();
+    const clean = mod._validateCustomMapping({ '38': 'snare', '200': 'kick', '40': 'nope-lane' });
+    assert.deepEqual({ ...clean }, { 38: 'snare' }); // clean has a null prototype
+});
+
+test('_validateCustomMapping rejects non-objects, arrays, and prototype-poisoning keys', () => {
+    const mod = freshPlugin();
+    assert.equal(mod._validateCustomMapping(null), null);
+    assert.equal(mod._validateCustomMapping('nope'), null);
+    assert.equal(mod._validateCustomMapping([1, 2]), null);
+    // Empty after filtering -> null, not {}.
+    assert.equal(mod._validateCustomMapping({ '__proto__': 'kick' }), null);
+    assert.equal(mod._validateCustomMapping({}), null);
+});
+
+test('_drumTabHitsToNotes converts known piece-ids to {t,s,f} and sorts by time', () => {
+    const mod = freshPlugin();
+    const notes = mod._drumTabHitsToNotes([
+        { p: 'snare', t: 1.0, v: 120 },
+        { p: 'kick', t: 0.5, v: 80 },
+    ]);
+    assert.equal(notes.length, 2);
+    assert.equal(notes[0].t, 0.5);
+    assert.equal(notes[0]._piece, 'kick');
+    assert.equal(notes[0].ac, false); // v=80 < 100
+    assert.equal(notes[1].ac, true);  // v=120 >= 100
+});
+
+test('_drumTabHitsToNotes drops unknown piece-ids and non-finite/negative timestamps', () => {
+    const mod = freshPlugin();
+    const notes = mod._drumTabHitsToNotes([
+        { p: 'cowbell', t: 1.0 },       // unknown piece
+        { p: 'snare', t: -1 },           // negative time
+        { p: 'snare', t: NaN },          // non-finite
+        { p: 'snare', t: 2.0 },          // kept
+    ]);
+    assert.equal(notes.length, 1);
+    assert.equal(notes[0].t, 2.0);
+});
+
+test('_drumTabHitsToNotes emits a leading grace note for flams', () => {
+    const mod = freshPlugin();
+    const notes = mod._drumTabHitsToNotes([{ p: 'snare', t: 1.0, v: 100, f: true }]);
+    assert.equal(notes.length, 2);
+    const [grace, main] = notes;
+    assert.equal(grace._noScore, true);
+    assert.ok(grace.t < main.t);
+    assert.equal(main.t, 1.0);
+});
+
+test('_drumTabHitsToNotes ignores a non-array payload', () => {
+    const mod = freshPlugin();
+    assert.deepEqual(mod._drumTabHitsToNotes(null), []);
+    assert.deepEqual(mod._drumTabHitsToNotes('nope'), []);
+});
+
+test('lane preset switch rebuilds DRUM_LANES and the default map', () => {
+    const mod = freshPlugin();
+    mod._applyLanePreset('rb4');
+    assert.deepEqual(mod.DRUM_LANES.map(l => l.id), ['hihat', 'snare', 'tom1', 'tom3', 'crash', 'ride', 'kick']);
+    // In rb4, mid-tom notes 45/47 fold into tom1 (no separate tom2 lane).
+    assert.equal(mod._midiToLaneIdx(45), mod.DRUM_LANES.findIndex(l => l.id === 'tom1'));
+
+    mod._applyLanePreset('phase_shift_8');
+    assert.deepEqual(mod.DRUM_LANES.map(l => l.id),
+        ['hihat', 'snare', 'tom1', 'tom2', 'tom3', 'crash', 'ride', 'kick']);
+});
+
+test('_applyLanePreset falls back to phase_shift_8 for an unknown preset name', () => {
+    const mod = freshPlugin();
+    mod._applyLanePreset('not-a-real-preset');
+    assert.deepEqual(mod.DRUM_LANES.map(l => l.id),
+        ['hihat', 'snare', 'tom1', 'tom2', 'tom3', 'crash', 'ride', 'kick']);
+});
+
+test('_midiToLaneIdx/_songNoteToLaneIdx resolve unmapped notes to -1', () => {
+    const mod = freshPlugin();
+    assert.equal(mod._midiToLaneIdx(999), -1);
+    assert.equal(mod._songNoteToLaneIdx(999), -1);
+});
+
+test('_midiResolveSaved matches by stored key first, falls back to legacy bare id', () => {
+    const mod = freshPlugin();
+    const sources = [{ id: 'dev1', key: 'webmidi:dev1' }];
+    assert.equal(mod._midiResolveSaved('webmidi:dev1', sources), 'webmidi:dev1');
+    assert.equal(mod._midiResolveSaved('dev1', sources), 'webmidi:dev1');
+    assert.equal(mod._midiResolveSaved('nope', sources), null);
+});
+
+test('matchesArrangement trusts has_drum_tab regardless of arrangement name', () => {
+    const mod = freshPlugin();
+    assert.equal(mod.matchesArrangement({ has_drum_tab: true, arrangement: 'Lead' }), true);
+});
+
+test('matchesArrangement matches drum-pattern arrangement names', () => {
+    const mod = freshPlugin();
+    assert.equal(mod.matchesArrangement({ arrangement: 'Drums' }), true);
+    assert.equal(mod.matchesArrangement({ arrangement: 'Lead Guitar' }), false);
+});
+
+test('matchesArrangement rejects falsy songInfo', () => {
+    const mod = freshPlugin();
+    assert.equal(mod.matchesArrangement(null), false);
+});
+
+test('PIECE_DEFAULT_MIDI covers the canonical piece set used by _drumTabHitsToNotes', () => {
+    const mod = freshPlugin();
+    assert.equal(mod.PIECE_DEFAULT_MIDI.kick, 36);
+    assert.equal(mod.PIECE_DEFAULT_MIDI.snare, 38);
+});