diff --git a/apps/desktop/src/main/__tests__/browser-panel-layout.test.ts b/apps/desktop/src/main/__tests__/browser-panel-layout.test.ts
new file mode 100644
index 00000000000..a8828e104bb
--- /dev/null
+++ b/apps/desktop/src/main/__tests__/browser-panel-layout.test.ts
@@ -0,0 +1,76 @@
+/**
+ * CSS contract test for BrowserPanel narrow layout (#546 closeout).
+ *
+ * Narrow source contract: locks ONLY the declaration that prevents the
+ * disappearing-browser defect — `min-height` on the narrow `.maka-browser-panel`
+ * rule. BrowserPanel's strip reports its rect to main each animation frame;
+ * without a panel min-height the strip's `flex: 1 1 auto` resolves to ~0 in the
+ * auto grid row and the native WebContentsView is mirrored to nothing, so the
+ * embedded browser disappears. `min-height` is genuinely exclusive to the
+ * narrow rule (the base `.maka-browser-panel` rule does not declare it), so a
+ * source contract can lock its effective value within the block.
+ *
+ * This does NOT simulate the CSS cascade and does NOT claim to lock width,
+ * border-left, max-height, border-top, or appearance. `width` / `border-left`
+ * are also declared by the base rule and the narrow values win only by source
+ * order, which a text contract cannot verify; `max-height` is a space cap, not
+ * the disappearance-prevention floor; `border-top` / `box-shadow` are visual.
+ * Those belong to visual smoke review, not a text contract. Locking the full
+ * computed layout would need a real browser computed-style check.
+ */
+import { strict as assert } from 'node:assert';
+import { readFile } from 'node:fs/promises';
+import { resolve } from 'node:path';
+import { describe, it } from 'node:test';
+import postcss from 'postcss';
+import { REPO_ROOT, stripCssComments } from './css-test-helpers.js';
+
+const CHAT_DETAIL_CSS = resolve(REPO_ROOT, 'apps/desktop/src/renderer/styles/chat-detail.css');
+
+interface MinHeightDecl {
+  value: string;
+  important: boolean;
+}
+
+/** All `min-height` declarations on exact-`.maka-browser-panel` rules inside
+ *  `@media (max-width: 990px)` blocks, in source order with their !important
+ *  flag. Pure over a CSS string so the collection/guard logic is unit-testable. */
+function narrowBrowserMinHeights(css: string): MinHeightDecl[] {
+  const root = postcss.parse(stripCssComments(css), { from: CHAT_DETAIL_CSS });
+  const decls: MinHeightDecl[] = [];
+  root.walkAtRules('media', (atRule) => {
+    if (!/^\(\s*max-width\s*:\s*990px\s*\)$/.test(atRule.params)) return;
+    atRule.walkRules((rule) => {
+      if (!rule.selectors.includes('.maka-browser-panel')) return;
+      for (const node of rule.nodes) {
+        if (node.type !== 'decl' || node.prop !== 'min-height') continue;
+        const d = node as postcss.Declaration;
+        decls.push({ value: d.value.trim(), important: Boolean(d.important) });
+      }
+    });
+  });
+  return decls;
+}
+
+describe('BrowserPanel narrow layout CSS contract', () => {
+  it('reserves a usable non-zero strip height at <=990px via a single unchallenged min-height', async () => {
+    const decls = narrowBrowserMinHeights(await readFile(CHAT_DETAIL_CSS, 'utf8'));
+    assert.equal(decls.length, 1, 'expected exactly one min-height on .maka-browser-panel in the 990px block (a second rule or duplicate property would compete)');
+    assert.equal(decls[0].value, '220px', 'expected min-height:220px so the strip (and native view) is non-zero');
+    assert.equal(decls[0].important, false, 'expected the narrow min-height not to need !important (a !important override would escape the contract)');
+  });
+
+  it('collects min-height across all matching rules so the guard catches a second overriding rule, a duplicate property, !important, or removal', () => {
+    const block = (browserRule: string) => `@media (max-width: 990px){.maka-artifact-pane{width:100%}${browserRule}}`;
+    // one clean declaration -> a single entry (the integration guard passes)
+    assert.equal(narrowBrowserMinHeights(block('.maka-browser-panel{min-height:220px}')).length, 1);
+    // a second overriding rule -> two entries (the integration length===1 guard fails)
+    assert.equal(narrowBrowserMinHeights(block('.maka-browser-panel{min-height:220px}.maka-browser-panel{min-height:0}')).length, 2);
+    // a duplicate property within one rule -> two entries (the guard fails)
+    assert.equal(narrowBrowserMinHeights(block('.maka-browser-panel{min-height:220px;min-height:0}')).length, 2);
+    // a !important -> recorded with the flag (the integration important===false guard fails)
+    assert.equal(narrowBrowserMinHeights(block('.maka-browser-panel{min-height:220px !important}'))[0].important, true);
+    // removal -> zero entries (the guard fails)
+    assert.equal(narrowBrowserMinHeights(block('.maka-browser-panel{width:100%}')).length, 0);
+  });
+});
\ No newline at end of file
diff --git a/apps/desktop/src/renderer/styles/chat-detail.css b/apps/desktop/src/renderer/styles/chat-detail.css
index 5b84deca3dc..f93851d9794 100644
--- a/apps/desktop/src/renderer/styles/chat-detail.css
+++ b/apps/desktop/src/renderer/styles/chat-detail.css
@@ -632,15 +632,37 @@
   background: oklch(from var(--destructive) l c h / 0.10);
 }
 
-/* Narrow window: ArtifactPane becomes a bottom sheet below the composer
-   instead of continuing to shrink the chat column. This keeps the send path
-   visible and usable while preserving artifact access on small windows. */
+/* Narrow window: ArtifactPane and BrowserPanel become bottom sheets
+   below the composer instead of continuing to squeeze the chat column,
+   preserving artifact and embedded-browser access on small windows.
+
+   BrowserPanel previously had no narrow rule. Its grid auto row sized to the
+   panel's max-content (toolbar height): the strip's `flex: 1 1 auto`
+   contributes 0 height without a definite container height, so the rAF loop
+   mirrored a ~zero-height rect to main and the native WebContentsView was
+   sized to nothing — the embedded browser disappeared beside a ~470px gap.
+   `min-height` below forces the auto row (and thus the strip) to a usable,
+   bounded size; `width: 100%` turns the 520px right rail into a bottom sheet.
+   BrowserPanel is always the second grid child (chat / browser / artifact),
+   so it already occupies the explicit `auto` row; ArtifactPane as the third
+   child uses an implicit `auto` row (grid-auto-rows defaults to auto), so no
+   extra explicit row is needed. With both auxiliary panels open on a very
+   short window the chat column can collapse; the user recovers by collapsing
+   ArtifactPane or closing the browser (tracked in the PR notes). */
 @media (max-width: 990px) {
   .maka-detail-with-artifacts {
     display: grid;
     grid-template-rows: minmax(0, 1fr) auto;
   }
 
+  .maka-browser-panel {
+    width: 100%;
+    min-height: 220px;
+    max-height: min(42dvh, 360px);
+    border-left: 0;
+    border-top: var(--border-width-hairline) solid var(--border);
+    box-shadow: 0 -8px 24px oklch(from var(--foreground) l c h / 0.08);
+  }
   .maka-artifact-pane {
     width: 100%;
     min-height: 220px;