fix(code-block): use absolute positioning for action buttons to fix clicks in nested scroll containers

[sleitor]

Problem

When streamdown is embedded in a layout with two or more nested scroll containers (e.g., an outer overflow-auto sidebar + an inner overflow-auto chat scroll area), the code block copy/download buttons are visible but unclickable.

The root cause: the sticky + pointer-events-none/auto pattern for the action button wrapper causes browser hit-testing to resolve clicks to the code block container div instead of reaching the buttons when there are multiple overflow: auto ancestors.

Fixes #494

Solution

• Switch the action button wrapper from position: sticky to position: absolute with top-2 right-2
• Add position: relative to the CodeBlockContainer so absolute positioning is scoped to the code block

This makes buttons reliably clickable in all scroll-container configurations.

Trade-off: Buttons no longer scroll with long code blocks (they stay fixed at the top-right of the code block). This is the same behavior as the workaround documented in the issue, and arguably cleaner UX — the buttons are always visible and always reachable.

Changes

• packages/streamdown/lib/code-block/container.tsx: add relative class
• packages/streamdown/lib/code-block/index.tsx: change outer wrapper from pointer-events-none sticky top-2 z-10 -mt-10 flex h-8 items-center justify-end to pointer-events-none absolute top-2 right-2 z-10 flex items-center
• .changeset/fix-code-block-buttons-absolute.md: patch changeset

Testing

All 982 existing tests pass (pnpm --filter streamdown test). Biome check clean.

[vercel]

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[vercel]

Additional Suggestion:

Mermaid block action buttons use sticky positioning that causes click event issues in nested scroll containers, identical to the bug fixed in code blocks.