diff --git a/web/e2e/bug-3420-selected-tab-contrast.spec.ts b/web/e2e/bug-3420-selected-tab-contrast.spec.ts
new file mode 100644
index 000000000..9d9ad863a
--- /dev/null
+++ b/web/e2e/bug-3420-selected-tab-contrast.spec.ts
@@ -0,0 +1,82 @@
+import { test, expect, type Page } from '@playwright/test';
+import { asNewUser, openTabs, show } from './lib/workspace-tabs';
+
+/**
+ * BUG-3420: in dark mode the selected workspace tab was hard to tell from the
+ * others. Its background is the page colour (the tab flows into the page), and
+ * that measured 1.07:1 against the strip and its neighbours in dark mode
+ * (1.09:1 in light), with the same text colour and weight. The fix is an accent
+ * line along the active tab's top edge, an inset box-shadow.
+ *
+ * This pins the requirement, not the style: in BOTH colour schemes the active
+ * tab carries an indicator colour that reaches WCAG 1.4.11's 3:1 against an
+ * inactive tab's background AND the strip's. Computed in the browser from the
+ * rendered colours, so a token change that sinks the contrast fails here.
+ */
+
+type Ratios = { indicator: string | null; vsInactive: number; vsStrip: number };
+
+async function activeTabIndicator(page: Page): Promise<Ratios> {
+	return page.evaluate(() => {
+		const parse = (c: string) => {
+			const m = c.match(/rgba?\(([^)]+)\)/);
+			if (!m) return null;
+			const p = m[1].split(/[ ,/]+/).filter(Boolean).map(Number);
+			return { r: p[0], g: p[1], b: p[2], a: p.length > 3 ? p[3] : 1 };
+		};
+		type C = NonNullable<ReturnType<typeof parse>>;
+		const lum = ({ r, g, b }: C) => {
+			const f = (v: number) => ((v /= 255) <= 0.03928 ? v / 12.92 : ((v + 0.055) / 1.055) ** 2.4);
+			return 0.2126 * f(r) + 0.7152 * f(g) + 0.0722 * f(b);
+		};
+		const ratio = (a: C, b: C) => {
+			const [x, y] = [lum(a), lum(b)].sort((p, q) => q - p);
+			return (x + 0.05) / (y + 0.05);
+		};
+		// The background an element shows: blend up to the first opaque ancestor.
+		const effBg = (el: Element | null): C => {
+			const stack: C[] = [];
+			for (let n = el; n; n = n.parentElement) {
+				const c = parse(getComputedStyle(n).backgroundColor);
+				if (c && c.a > 0) {
+					stack.push(c);
+					if (c.a >= 1) break;
+				}
+			}
+			let acc: C = { r: 0, g: 0, b: 0, a: 1 };
+			for (let i = stack.length - 1; i >= 0; i--) {
+				const t = stack[i];
+				acc = { r: t.r * t.a + acc.r * (1 - t.a), g: t.g * t.a + acc.g * (1 - t.a), b: t.b * t.a + acc.b * (1 - t.a), a: 1 };
+			}
+			return acc;
+		};
+		const active = document.querySelector('.workspace-tab.active')!;
+		const inactive = document.querySelector('.workspace-tab:not(.active)')!;
+		const strip = active.parentElement!;
+		const shadow = getComputedStyle(active).boxShadow;
+		const ind = shadow === 'none' ? null : parse(shadow);
+		if (!ind || ind.a < 1) return { indicator: null, vsInactive: 0, vsStrip: 0 };
+		return {
+			indicator: shadow,
+			vsInactive: +ratio(ind, effBg(inactive)).toFixed(2),
+			vsStrip: +ratio(ind, effBg(strip)).toFixed(2)
+		};
+	});
+}
+
+for (const colorScheme of ['dark', 'light'] as const) {
+	test(`BUG-3420: the active workspace tab has a 3:1 indicator (${colorScheme})`, async ({ page }, testInfo) => {
+		test.skip(testInfo.project.name !== 'desktop-chromium', 'the tab bar is desktop only');
+		await page.emulateMedia({ colorScheme });
+		const { username, slugs } = await asNewUser(page);
+		await openTabs(page, slugs, 3);
+		await show(page, username, slugs[1], 3);
+		await expect
+			.poll(() => page.evaluate(() => document.documentElement.getAttribute('data-theme')))
+			.toBe(colorScheme === 'light' ? 'light' : null);
+		const r = await activeTabIndicator(page);
+		expect(r.indicator, 'the active tab carries no solid indicator').not.toBeNull();
+		expect(r.vsInactive, `indicator vs an inactive tab (${r.indicator})`).toBeGreaterThanOrEqual(3);
+		expect(r.vsStrip, `indicator vs the strip (${r.indicator})`).toBeGreaterThanOrEqual(3);
+	});
+}
diff --git a/web/src/lib/components/items/ItemDetail.svelte b/web/src/lib/components/items/ItemDetail.svelte
index ecd35ec57..a1c3f1bfe 100644
--- a/web/src/lib/components/items/ItemDetail.svelte
+++ b/web/src/lib/components/items/ItemDetail.svelte
@@ -8391,10 +8391,12 @@
 		color: var(--text-secondary);
 	}
 
+	/* Selected segment: a 1px accent ring (BUG-3420). Background alone measured
+	   1.05-1.16:1 against its neighbours in both themes; the ring is >= 4.4:1. */
 	.mode-btn.active {
 		background: var(--bg-secondary);
 		color: var(--text-primary);
-		box-shadow: 0 1px 2px rgba(0, 0, 0, 0.1);
+		box-shadow: inset 0 0 0 1px var(--accent-primary);
 	}
 
 	/* Code context */
diff --git a/web/src/lib/components/layout/TopBar.svelte b/web/src/lib/components/layout/TopBar.svelte
index 9fc97f865..78cdc9a98 100644
--- a/web/src/lib/components/layout/TopBar.svelte
+++ b/web/src/lib/components/layout/TopBar.svelte
@@ -1028,9 +1028,18 @@
 	.workspace-tab:hover {
 		background: var(--bg-hover);
 	}
+	/* The selected tab carries a 2px accent line along its top edge (BUG-3420).
+	   Its background alone could not mark it: the active tab flows into the
+	   page, so it IS the page colour, and that measured 1.07:1 (dark) and
+	   1.09:1 (light) against the strip and its neighbours, with the same text
+	   colour and weight. The line is WCAG non-text contrast against both: in
+	   dark #9268f8 is 4.85:1 on the strip and 5.20:1 on the tab, in light
+	   #7c3aed is 5.70:1 and 5.24:1. Inset, so it follows the rounded corners
+	   and moves nothing. */
 	.workspace-tab.active {
 		background: var(--bg-primary);
 		border-color: var(--border);
+		box-shadow: inset 0 2px 0 var(--accent-primary);
 		z-index: 1;
 	}
 	.workspace-tab-close {
diff --git a/web/src/routes/[username]/[workspace]/insights/+page.svelte b/web/src/routes/[username]/[workspace]/insights/+page.svelte
index 001f93eb1..d755b4a27 100644
--- a/web/src/routes/[username]/[workspace]/insights/+page.svelte
+++ b/web/src/routes/[username]/[workspace]/insights/+page.svelte
@@ -848,10 +848,12 @@
 	.window-btn:hover {
 		color: var(--text-primary);
 	}
+	/* Selected segment: a 1px accent ring (BUG-3420). Background alone measured
+	   1.05-1.16:1 against its neighbours in both themes; the ring is >= 4.4:1. */
 	.window-btn.active {
 		background: var(--bg-primary, var(--bg));
 		color: var(--text-primary);
-		box-shadow: 0 1px 2px rgba(0, 0, 0, 0.08);
+		box-shadow: inset 0 0 0 1px var(--accent-primary);
 	}
 
 	/* ── Print report link ────────────────────────────────────────────── */
diff --git a/web/src/routes/[username]/[workspace]/tags/[tag]/+page.svelte b/web/src/routes/[username]/[workspace]/tags/[tag]/+page.svelte
index aafc94048..ac0eba189 100644
--- a/web/src/routes/[username]/[workspace]/tags/[tag]/+page.svelte
+++ b/web/src/routes/[username]/[workspace]/tags/[tag]/+page.svelte
@@ -287,10 +287,13 @@
 		border-left: 1px solid var(--border);
 	}
 
+	/* Selected segment: a 1px accent ring (BUG-3420). Background alone measured
+	   1.05-1.16:1 against its neighbours in both themes; the ring is >= 4.4:1. */
 	.view-btn.active {
 		background: var(--bg-tertiary);
 		color: var(--text-primary);
 		font-weight: 600;
+		box-shadow: inset 0 0 0 1px var(--accent-primary);
 	}
 
 	.tag-board {