How to make position: sticky work inside a CSS Grid child with overflow?

I am trying to create a layout where the page height is fixed to the viewport (100vh), the .sidebar and .content areas scroll independently, and a .content-header inside .content should stay sticky while the .content-body scrolls.

Here is the CSS I am using:

body {
  margin: 0;
  height: 100vh;
  display: flex;
  flex-direction: column;
}

.page-header {
  flex: 0 0 auto;
  padding: 1rem;
  background: #333;
  color: white;
}

.layout {
  flex: 1 1 auto;
  display: grid;
  grid-template-columns: 250px 1fr;
  max-height: 100%;
  overflow: hidden; /* needed so main area does not scroll the whole page */
}

.sidebar {
  overflow-y: auto;
  padding: 1rem;
  background: #f1f1f1;
}

.content {
  overflow-y: auto; /* scroll only this area */
  padding: 1rem;
}

/* Sticky header inside the scrollable content area */
.content-header {
  position: sticky;
  top: 0;
  padding: 0.5rem 0;
  background: white;
  border-bottom: 1px solid #ccc;
  z-index: 1;
}

.content-body {
  padding-top: 1rem;
}

Expected behavior:

• The whole page height is fixed to the viewport (100vh);

• Only .sidebar and .content should scroll;

• Inside .content, the .content-header should remain sticky at the top while .content-body scrolls underneath it.

However, position: sticky is not working inside the grid layout with overflow: hidden.

Why is this happening, and how can I keep the header sticky while allowing the content to scroll?