fix(streamdown): re-render memoized components when node offset changes

[julesyoungberg]

Description

Fixes stale DOM when markdown is updated in the middle of an existing block. Memoized leaf components (ol, ul, p, headings, etc.) use sameNodePosition to skip re-renders, but it only compared line and column. When content changed without updating line/column (only offset, e.g. injecting a link mid-line), React kept stale output until Streamdown remounted.

sameNodePosition is shared by all memoized markdown components via sameClassAndNode, so this fix applies broadly rather than to a single element type.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Changes Made

• Added offset?: number to MarkdownPoint and compare position.start.offset / position.end.offset in sameNodePosition (packages/streamdown/lib/components.tsx)
• Added regression test "should re-render when only offset changes" (packages/streamdown/__tests__/components-rerender.test.tsx)
• Added patch changeset (.changeset/fair-balloons-share.md)

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

pnpm exec vitest run __tests__/components-rerender.test.tsx

All 18 tests in that file pass.

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Additional Notes

No documentation or inline comment changes — this is an internal comparator fix with no API surface change. The new test documents the edge case.

[vercel]

@julesyoungberg is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.

[dasheidt]

This fixes the case where content grows, but there is a case it cannot reach: a replacement of the same length moves no offset either.

const { container, rerender } = render(<Streamdown>{"Revenue grew"}</Streamdown>);
rerender(<Streamdown>{"Booking grew"}</Streamdown>);   // same length, same offsets

container.textContent; // "Revenue grew" — with or without this PR

start.offset is 0 and end.offset is 12 before and after, so the comparator still reports "unchanged" and the paragraph keeps rendering the old text. Same for a heading, a list item or a <strong> run of unchanged length.

The reason offset does not close it is that any position — line, column or offset — describes where the text came from, not what the component renders. I opened #584, which replaces the position comparison with React's own shallow prop comparison minus node, so the same test that motivated this PR passes, plus the same-length case and #570.

Not trying to steamroll this one — if the maintainers prefer the narrower change, the offset fix here is strictly better than what is on main today.

[farnabaz]

Thanks for PR @julesyoungberg, I'm closing this in favor of #584.