fix(markdown): remove extra whitespace from container markup

__tests__/unit/node/markdown/plugins/containers.test.ts

@@ -223,6 +223,22 @@ describe('node/markdown/plugins/containers', () => {
     `)
   })
 
+  // FIXME: enable after attrs plugin version bump
+  test.skip('respects attrs plugin options on the fence line', async () => {
+    const delimiters = await render(
+      '::: tip Custom %(.extra-class)%\ncontent\n:::',
+      { attrs: { left: '%(', right: ')%' } }
+    )
+    expect(delimiters).toContain('<div class="extra-class tip custom-block">')
+
+    const allowed = await render(
+      '::: tip Custom {.extra-class data-x=1}\ncontent\n:::',
+      { attrs: { allowed: ['class'] } }
+    )
+    expect(allowed).toContain('<div class="extra-class tip custom-block">')
+    expect(allowed).not.toContain('data-x')
+  })
+
   test('keeps fence line braces verbatim when attrs are disabled', async () => {
     expect(
       await render('::: details Click me {open}\ncontent\n:::', {

src/node/markdown/plugins/containers.ts

@@ -138,7 +138,7 @@ function createOpenRender(
     // details always needs its summary, so no-title is ignored there
     const noTitle = attrPop(token, 'no-title') === '' && name !== 'details'
     token.attrJoin('class', `${name} custom-block`)
-    const renderedAttrs = md.renderer.renderAttrs(token)
+    const renderedAttrs = md.renderer.renderAttrs(token).trim()
     if (noTitle) return `<div ${renderedAttrs}>\n`
     const title = md.renderInline(
       info || titlesFor(titles, env.localeIndex)[name],