Diff View

A line-by-line diff of two strings — the before/after view a screen needs when it has to show what changed: a config or settings change, a record edited in an admin panel, a document revision, a webhook payload against the last one, an audit-log entry, a restored backup next to what is live, or the edit an AI agent is proposing before the user accepts it. Pass before and after and it renders a git-style diff, unified by default or side by side with view="split". Unchanged lines collapse into a counted gap, so a 400-line file with a three-line change shows three lines and a summary instead of 400; context sets how many surrounding lines survive and context={Infinity} shows the whole text. Both line-number gutters are select-none, so selecting the diff copies the code and not a column of numbers. CRLF and LF are folded together, because a file that changed only its line endings would otherwise report every single line as rewritten. It is meaning-first rather than colour-first: every changed row carries a + or - sign and a screen-reader-only "Added line:" / "Removed line:" prefix, so the diff still reads for someone who cannot tell the red and green backgrounds apart — conveying the change by colour alone, which fails WCAG 1.4.1, is the single most common defect in a hand-rolled diff. The table also gets an sr-only caption stating how many lines were added and removed. The diff is a longest-common-subsequence over lines with the shared prefix and suffix trimmed off first, which keeps a large document with a small edit fast (a 4,000-line file with one changed line diffs in well under a millisecond) and makes an appended line read as appended instead of shifting everything by one. Pathologically large inputs degrade to "this block was replaced" rather than allocating a table of hundreds of megabytes during a render. No dependencies, no diff library and no hooks, so it renders inside a React server component without a "use client" of its own and ships no client JavaScript — which is the common case, because the text being compared has usually just been fetched on the server. Official shadcn/ui has no diff component of any kind: table is an unstyled table and chart is a Recharts wrapper, and neither computes or displays a change.