diff --git a/app/igneum-app/ui/app.css b/app/igneum-app/ui/app.css
index ca4f63c7b..cd608c44b 100644
--- a/app/igneum-app/ui/app.css
+++ b/app/igneum-app/ui/app.css
@@ -474,18 +474,21 @@ table{border-collapse:collapse;width:100%;font-size:var(--t-base)}
 .row{display:flex;gap:10px;align-items:center;min-width:0}
 .row.wrap{flex-wrap:wrap}
 .row .addr-input{margin-top:0;min-width:0}
-.switch{display:flex;align-items:flex-start;gap:var(--s-3);font-size:var(--t-md);cursor:pointer;padding:9px 0;line-height:1.4}
+/* the custom switch (Prove page fix, 7 October 2026): the label is the positioned ancestor, the native input is hidden
+   the accessible way (1 px, clipped, still focusable, the label association kept), the knob lives inside the track at
+   both states; the size class is the switch's own (big), never the DAG legend's swatch class */
+.switch{display:flex;align-items:flex-start;gap:var(--s-3);font-size:var(--t-md);cursor:pointer;padding:9px 0;line-height:1.4;position:relative}
 .switch+.switch{border-top:1px solid var(--line)}
-.switch input{position:absolute;opacity:0;width:0;height:0}
+.switch input{position:absolute;width:1px;height:1px;margin:-1px;padding:0;border:0;overflow:hidden;clip:rect(0 0 0 0);clip-path:inset(50%);white-space:nowrap}
 .switch .track{width:40px;height:22px;border-radius:999px;background:var(--line-2);position:relative;flex:0 0 40px;transition:background .15s ease;margin-top:1px}
 .switch .track::after{content:"";position:absolute;top:3px;left:3px;width:16px;height:16px;border-radius:50%;background:#F4F1EC;transition:transform .15s ease}
 .switch input:checked+.track{background:var(--ember)}
 .switch input:checked+.track::after{transform:translateX(18px)}
 .switch input:focus-visible+.track{outline:2px solid var(--ember);outline-offset:3px}
 .switch input:disabled+.track{opacity:.4}
-.switch.lg .track{width:52px;height:30px;flex-basis:52px}
-.switch.lg .track::after{width:24px;height:24px}
-.switch.lg input:checked+.track::after{transform:translateX(22px)}
+.switch.big .track{width:52px;height:30px;flex-basis:52px}
+.switch.big .track::after{width:24px;height:24px}
+.switch.big input:checked+.track::after{transform:translateX(22px)}
 .sw-text{min-width:0}
 .sw-text b{font-weight:600}
 .sw-text .dim{font-size:var(--t-base)}
@@ -664,13 +667,14 @@ body.drawer-open .toast{bottom:calc(var(--drawer-h) + 16px)}
 .dag-chip{position:absolute;right:10px;top:10px;font-size:var(--t-xs);color:var(--ash);background:var(--graphite);border:1px solid var(--line-2);border-radius:999px;padding:3px 10px;pointer-events:none;z-index:2}
 .legend{display:flex;flex-wrap:wrap;gap:8px 16px;margin-top:var(--s-3);font-size:var(--t-sm);color:var(--ash);align-items:center}
 .legend span{display:inline-flex;align-items:center;gap:7px}
-.lg{width:10px;height:10px;border-radius:3px;display:inline-block;border:1.5px solid var(--ash);background:var(--row)}
-.lg.you{border-color:var(--molten);box-shadow:0 0 0 2px var(--molten-40)}
-.lg.pending{border-color:var(--ash)}
-.lg.included{border-color:var(--ember)}
-.lg.excluded{border-color:var(--ash);opacity:.45}
-.lg.proven{background:var(--ember);border-color:var(--ember)}
-.lg.locked{border-color:var(--ember);border-radius:50%;box-shadow:0 0 0 2px var(--ember-40)}
+/* the DAG legend swatches, scoped to the legend (a bare .lg reached label.switch.lg until 7 October 2026) */
+.legend .lg{width:10px;height:10px;border-radius:3px;display:inline-block;border:1.5px solid var(--ash);background:var(--row)}
+.legend .lg.you{border-color:var(--molten);box-shadow:0 0 0 2px var(--molten-40)}
+.legend .lg.pending{border-color:var(--ash)}
+.legend .lg.included{border-color:var(--ember)}
+.legend .lg.excluded{border-color:var(--ash);opacity:.45}
+.legend .lg.proven{background:var(--ember);border-color:var(--ember)}
+.legend .lg.locked{border-color:var(--ember);border-radius:50%;box-shadow:0 0 0 2px var(--ember-40)}
 /* Cards: the Tuning strip above the list */
 .tune-strip{padding:var(--s-4) var(--card-pad)}
 .ts-row{display:flex;align-items:center;gap:var(--s-4);flex-wrap:wrap}
@@ -799,9 +803,10 @@ body.drawer-open .toast{bottom:calc(var(--drawer-h) + 16px)}
 .proof-scene{height:180px;border:1px solid var(--line);border-radius:6px;background:var(--row);margin:10px 0;overflow:hidden}
 .proof-scene canvas{display:block;width:100%;height:100%}
 .proof-head{display:flex;justify-content:space-between;align-items:baseline;font-weight:500;color:var(--bone);margin-bottom:8px}
-.track{display:flex;gap:3px;height:3px;margin-bottom:10px}
-.track i{flex:1 1 0;background:var(--line);border-radius:2px}
-.track i.proving{background:var(--ember)}.track i.verified{background:var(--bone)}.track i.paid{background:var(--molten)}
+/* the inspector's shard track, scoped to its element (a bare .track reached every switch's track until 7 October 2026) */
+#i-track{display:flex;gap:3px;height:3px;margin-bottom:10px}
+#i-track i{flex:1 1 0;background:var(--line);border-radius:2px}
+#i-track i.proving{background:var(--ember)}#i-track i.verified{background:var(--bone)}#i-track i.paid{background:var(--molten)}
 .shards{list-style:none;margin:0 0 12px;padding:0;font-size:var(--t-xs);line-height:1.7;max-height:120px;overflow:auto}
 .shards li{display:flex;justify-content:space-between;gap:8px;color:var(--ink-2)}
 .shards li::before{content:"\25CF";font-size:7px;margin-right:6px;color:var(--line-2)}
diff --git a/app/igneum-app/ui/index.html b/app/igneum-app/ui/index.html
index 4e74d13c2..3e05e8f9f 100644
--- a/app/igneum-app/ui/index.html
+++ b/app/igneum-app/ui/index.html
@@ -429,7 +429,7 @@
             <h3>Prove on this machine</h3>
             <p class="help">Every block is turned into a short proof, in pieces called shards. Your cards prove shards and earn IGN for each one.</p>
           </div>
-          <label class="switch lg" title="Prove on this machine"><input type="checkbox" id="s-prove" aria-label="Prove on this machine"><span class="track"></span></label>
+          <label class="switch big" title="Prove on this machine"><input type="checkbox" id="s-prove" aria-label="Prove on this machine"><span class="track"></span></label>
         </div>
         <ul class="tier-list" id="pv-tiers"></ul>
         <p class="line-text" id="pv-line"></p>
diff --git a/app/igneum-app/ui/view.test.mjs b/app/igneum-app/ui/view.test.mjs
index 216680f2b..a67e94c8a 100644
--- a/app/igneum-app/ui/view.test.mjs
+++ b/app/igneum-app/ui/view.test.mjs
@@ -811,3 +811,49 @@ test('the light theme contrast ratio of every vendor colour on its well is at le
   assert.equal(/\.badge\.(nvidia|amd|intel|apple|gpu)\{[^}]*var\(--ember/.test(css), false, 'no badge rule tints with ember');
   assert.equal(/\.badge[^{]*\{[^}]*animation/.test(css), false, 'nothing on the mark animates');
 });
+
+// ---------- the custom switch (the Prove page fix, 7 October 2026) ----------
+// the project lead's screenshot from the live 0.3.19 Mac app: a thin white rectangle above-left of the "Prove on this machine"
+// switch and the knob drawn outside the track's left edge at off. Two generic classes reached the switch (the DAG
+// legend swatch .lg hit label.switch.lg, the shard track .track hit .switch .track) and the native input was an
+// unpositioned absolute. This test reads app.css: no bare .lg or .track rule may exist (every use is scoped), the
+// native input is hidden the accessible way (1 px clip, still focusable), and the knob's box sits inside the track's
+// box at off and at on for both sizes, in the base rules and untouched by both theme blocks.
+const cssRules = (text) => { const out = []; const re = /([^{}]+)\{([^{}]*)\}/g; let m; text = text.replace(/\/\*[\s\S]*?\*\//g, ''); while ((m = re.exec(text))) out.push({ sel: m[1].trim(), body: m[2] }); return out; };
+const ruleOf = (sel) => cssRules(css).filter((r) => r.sel.split(',').map((x) => x.trim()).includes(sel)).map((r) => r.body).join(';');
+const px = (body, prop) => { const m = new RegExp('(?:^|;)\\s*' + prop + ':\\s*(-?[\\d.]+)px').exec(body); return m ? parseFloat(m[1]) : null; };
+const translate = (body) => { const m = /transform:\s*translateX\((-?[\d.]+)px\)/.exec(body); return m ? parseFloat(m[1]) : 0; };
+
+test('no generic .lg or .track rule reaches a custom switch: every use of those class names is scoped (known-failed on the 0.3.19 CSS)', () => {
+  const bare = cssRules(css).flatMap((r) => r.sel.split(',').map((x) => x.trim())).filter((sel) => /^\.(lg|track)(\.[\w-]+)*(\s+[\w.:-]+)*$/.test(sel) && !/^\.switch\b/.test(sel));
+  assert.deepEqual(bare, [], 'selectors that start with a bare .lg or .track: ' + bare.join(' | '));
+  assert.equal(/<label class="switch lg"/.test(readFileSync(join(dirname(fileURLToPath(import.meta.url)), 'index.html'), 'utf8')), false, 'the Prove switch no longer shares the legend swatch class');
+});
+
+test('the native input of every custom switch is hidden the accessible way: 1 px clipped, still in the tree and focusable, the label kept', () => {
+  const sw = ruleOf('.switch'), input = ruleOf('.switch input');
+  assert.match(sw, /position:\s*relative/, '.switch is the positioned ancestor of its input');
+  assert.match(input, /position:\s*absolute/);
+  assert.equal(px(input, 'width'), 1); assert.equal(px(input, 'height'), 1);
+  assert.match(input, /clip:\s*rect\(0 0 0 0\)|clip-path:\s*inset\(50%\)/, 'the clip pattern');
+  assert.match(input, /overflow:\s*hidden/);
+  assert.equal(/display:\s*none|visibility:\s*hidden/.test(input), false, 'display none would take the input out of the tab order');
+  assert.match(ruleOf('.switch input:focus-visible+.track'), /outline/, 'focus stays visible on the track');
+});
+
+test('the knob sits inside the track at off and at on, both sizes, and neither theme block touches the switch geometry', () => {
+  const track = ruleOf('.switch .track'), knob = ruleOf('.switch .track::after'), on = ruleOf('.switch input:checked+.track::after');
+  const big = ruleOf('.switch.big .track'), bigKnob = ruleOf('.switch.big .track::after'), bigOn = ruleOf('.switch.big input:checked+.track::after');
+  const inside = (tw, th, kw, kh, left, top, dx, label) => {
+    assert.ok(left >= 0 && top >= 0, label + ': the knob starts inside (left ' + left + ', top ' + top + ')');
+    assert.ok(left + kw <= tw && top + kh <= th, label + ' off: knob ' + kw + 'x' + kh + ' at ' + left + ',' + top + ' in a ' + tw + 'x' + th + ' track');
+    assert.ok(left + dx + kw <= tw, label + ' on: knob right edge ' + (left + dx + kw) + ' in a ' + tw + ' track');
+    assert.ok(left + dx >= 0, label + ' on: knob left edge ' + (left + dx));
+  };
+  assert.match(track, /position:\s*relative/, 'the track positions its knob');
+  inside(px(track, 'width'), px(track, 'height'), px(knob, 'width'), px(knob, 'height'), px(knob, 'left'), px(knob, 'top'), translate(on), 'the 40 px switch');
+  inside(px(big, 'width'), px(big, 'height'), px(bigKnob, 'width'), px(bigKnob, 'height'), px(knob, 'left'), px(knob, 'top'), translate(bigOn), 'the big switch');
+  // the two theme blocks (prefers-color-scheme light, data-theme light) only set tokens: no .switch or .track rule inside them
+  for (const m of css.matchAll(/@media \(prefers-color-scheme:light\)\{:root:not\(\[data-theme="dark"\]\)\{[^}]*\}\}|:root\[data-theme="light"\]\{[^}]*\}/g)) assert.equal(/\.switch|\.track/.test(m[0]), false, 'a theme block touches the switch');
+  assert.equal(cssRules(css).some((r) => /\.switch|\.track/.test(r.sel) && /@media \(prefers-color-scheme/.test(r.sel)), false);
+});