diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3233fb62a..4b1676c71 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -74,6 +74,7 @@
   snippet running under file-based execution.
 * Fixed the snippet editor reporting a save as failed and undelivered when no response was received, which it cannot
   determine.
+* Fixed a long unbroken line of code widening the snippet editor page and pushing the sidebar out of view.
 
 ## [3.10.2] (2026-09-01)
 
diff --git a/src/css/edit/_form.scss b/src/css/edit/_form.scss
index 99aebec60..165507559 100644
--- a/src/css/edit/_form.scss
+++ b/src/css/edit/_form.scss
@@ -90,7 +90,12 @@
 .snippet-form {
 	display: grid;
 	gap: var(--cs-sidebar-gap);
-	grid-template-columns: 1fr var(--cs-sidebar-width);
+
+	// The content column is bounded at zero rather than left as a bare 1fr,
+	// whose automatic minimum is its content width. A single long unbroken line
+	// of code would otherwise widen the track to fit, taking the sidebar off
+	// the side of the page with it.
+	grid-template-columns: minmax(0, 1fr) var(--cs-sidebar-width);
 	grid-template-areas: 'upper sidebar' 'lower sidebar';
 	transition: all 700ms;
 
@@ -115,6 +120,7 @@
 		display: flex;
 		flex-flow: column;
 		gap: 24px;
+		min-inline-size: 0;
 	}
 
 	.snippet-editor-sidebar {
@@ -134,7 +140,7 @@
 	}
 
 	@media (width <= 1024px) {
-		grid-template-columns: 1fr;
+		grid-template-columns: minmax(0, 1fr);
 	}
 
 	h2, h3 {
diff --git a/tests/e2e/code-snippets-edit.spec.ts b/tests/e2e/code-snippets-edit.spec.ts
index 2257461bb..f018b055c 100644
--- a/tests/e2e/code-snippets-edit.spec.ts
+++ b/tests/e2e/code-snippets-edit.spec.ts
@@ -15,6 +15,43 @@ test.describe('Code Snippets Admin', () => {
 		await helper.expectToBeOnSnippetsAdminPage()
 	})
 
+	test('A long unbroken line does not widen the editor page', async ({ page }) => {
+		const snippetName = SnippetsTestHelper.makeUniqueSnippetName()
+
+		// Saved through the editor rather than WP-CLI. CI runs WP-CLI as root, and
+		// with file-based execution on, the first snippet saved that way leaves
+		// root-owned directories that the web server cannot write later snippets
+		// into, so every snippet saved after it silently never runs.
+		await helper.createSnippet({
+			name: snippetName,
+			code: `// ${'a'.repeat(4000)}`
+		})
+
+		try {
+			await helper.openSnippet(snippetName)
+
+			// Line wrapping is on by default, and the page widened regardless.
+			await expect(page.locator('.CodeMirror').first()).toHaveClass(/CodeMirror-wrap/)
+
+			// One pixel of tolerance covers sub-pixel rounding in layout.
+			const pageOverflow = await page.evaluate(() =>
+				document.documentElement.scrollWidth - document.documentElement.clientWidth)
+			expect(pageOverflow, 'the page should not scroll sideways').toBeLessThanOrEqual(1)
+
+			const sidebar = await page.locator('.snippet-editor-sidebar').boundingBox()
+
+			if (!sidebar) {
+				throw new Error('The editor sidebar is not laid out')
+			}
+
+			const viewportWidth = page.viewportSize()?.width ?? 0
+			expect(sidebar.x + sidebar.width, 'the sidebar should stay on screen')
+				.toBeLessThanOrEqual(viewportWidth + 1)
+		} finally {
+			await helper.cleanupSnippet(snippetName)
+		}
+	})
+
 	test('Can add a new snippet', async () => {
 		const snippetName = SnippetsTestHelper.makeUniqueSnippetName()
 		await helper.createSnippet({