fix(theme): prevent code blocks painting over next paragraph

Pair overflow-x: auto with overflow-y: hidden so the wrapper does not
become a vertical scrollport. That stops WebKit painting the block and
its scrollbar over the following paragraph.

__tests__/e2e/markdown-extensions/markdown-extensions.test.ts

@@ -362,3 +362,37 @@ describe('Image Lazy Loading', () => {
     expect(await img.getAttribute('loading')).toBe('lazy')
   })
 })
+
+describe('Wide code blocks', () => {
+  beforeEach(async () => {
+    await goto('/markdown-extensions/wide-code')
+  })
+
+  test('do not paint over the following paragraph', async () => {
+    const block = page.locator('.vp-doc div[class*="language-"]').first()
+    const paragraph = page.locator('#after-wide-code')
+
+    expect(await block.count()).toBe(1)
+    expect(await paragraph.count()).toBe(1)
+
+    const overflowY = await block.evaluate(
+      (el) => getComputedStyle(el).overflowY
+    )
+    expect(overflowY).toBe('hidden')
+
+    const pre = block.locator('pre')
+    expect(await pre.evaluate((el) => el.scrollWidth > el.clientWidth)).toBe(
+      true
+    )
+
+    const [blockBox, paragraphBox] = await Promise.all([
+      block.boundingBox(),
+      paragraph.boundingBox()
+    ])
+    expect(blockBox).toBeTruthy()
+    expect(paragraphBox).toBeTruthy()
+    expect(paragraphBox!.y).toBeGreaterThanOrEqual(
+      blockBox!.y + blockBox!.height - 1
+    )
+  })
+})

__tests__/e2e/markdown-extensions/wide-code.md

@@ -0,0 +1,9 @@
+# Wide Code Block
+
+A paragraph before the code.
+
+```js
+const wide = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
+```
+
+<p id="after-wide-code">A paragraph after the wide code block.</p>

__tests__/unit/client/theme-default/code-overflow.test.ts

@@ -0,0 +1,31 @@
+import { readFileSync } from 'node:fs'
+import { fileURLToPath } from 'node:url'
+
+const vpDocCss = readFileSync(
+  fileURLToPath(
+    new URL(
+      '../../../../src/client/theme-default/styles/components/vp-doc.css',
+      import.meta.url
+    )
+  ),
+  'utf8'
+)
+
+function ruleFor(css: string, selector: string): string {
+  const start = css.indexOf(selector)
+  if (start === -1) {
+    throw new Error(`missing selector ${selector}`)
+  }
+  const open = css.indexOf('{', start)
+  const close = css.indexOf('}', open)
+  return css.slice(open + 1, close)
+}
+
+describe('client/theme-default/code-overflow', () => {
+  test('code block wrappers clip vertical overflow', () => {
+    const rule = ruleFor(vpDocCss, ".vp-doc div[class*='language-']")
+
+    expect(rule).toMatch(/overflow-x:\s*auto/)
+    expect(rule).toMatch(/overflow-y:\s*hidden/)
+  })
+})

src/client/theme-default/styles/components/vp-doc.css

@@ -354,7 +354,11 @@
   position: relative;
   margin: 1rem -1.5rem;
   background-color: var(--vp-code-block-bg);
+  /* pair overflow-x so overflow-y does not compute to auto; otherwise the
+     wrapper becomes a vertical scrollport and WebKit can paint the block
+     (and its scrollbar) over the following paragraph */
   overflow-x: auto;
+  overflow-y: hidden;
   transition: background-color 0.5s;
 }