Frames?

Frames are best avoided for site layout. They create separate scrolling contexts, break bookmarking and back-button behavior, hurt printing and accessibility, and complicate search indexing. The sidebar disappearing at the end of the page is exactly the kind of UX problem frames introduce: the sidebar sits in its own frame and does not participate in the main document flow. Building on @dickersonka and @jbennet, the modern, maintainable pattern is a single HTML document with a shared template or partial for the sidebar so the markup is edited in one place.

The "permission denied" error with iframes most often comes from browser security: same-origin policy or embedding being blocked by response headers (X-Frame-Options or Content-Security-Policy frame-ancestors). Useful diagnostics are the browser console and the network/response headers. Relevant references: Same-origin policy and X-Frame-Options / frame-ancestors (and the CSP variant).

Practical options: use server-side templates/partials (master layouts) or a lightweight static-site build that injects the sidebar at build time; if the host supports it, server-side includes are an easy, low-overhead option. For pure client-side approaches, fetch the shared HTML and inject it (same-origin only) and provide a server-rendered fallback for SEO/accessibility. For layout, Flexbox or Grid keeps the sidebar in the normal flow so it grows with content instead of being clipped. Example layout pattern (simplest form):

<div class="wrap">
  <aside class="sidebar">Sidebar</aside>
  <main class="content">Main</main>
</div>

<style>
.wrap { display:flex; align-items:flex-start; }
.sidebar { width:240px; }
.content { flex:1; }
</style>