fix(tui): preserve diff hunk boundaries (#39693)

Co-authored-by: James Long <17031+jlongster@users.noreply.github.com>

packages/tui/src/util/diff.ts

@@ -0,0 +1,56 @@
+export interface PatchHunk {
+  readonly patch: string
+  readonly header?: string
+  readonly rows?: number
+}
+
+export function splitPatchHunks(patch: string): PatchHunk[] {
+  const starts = [
+    ...patch.matchAll(/^@@ -\d+(?:,\d+)? \+\d+(?:,\d+)? @@.*$/gm),
+  ].map((match) => match.index)
+  if (starts.length <= 1) return [{ patch }]
+
+  const prefix = patch.slice(0, starts[0])
+  return starts.map((start, index) => {
+    const end = starts[index + 1] ?? patch.length
+    const lineEnd = patch.indexOf("\n", start)
+    return {
+      header: patch.slice(start, lineEnd === -1 ? end : lineEnd),
+      patch: prefix + patch.slice(start, end),
+      rows: splitRows(patch.slice(start, end)),
+    }
+  })
+}
+
+function splitRows(hunk: string) {
+  const lines = hunk.replace(/\n$/, "").split("\n").slice(1)
+  let rows = 0
+  let index = 0
+
+  while (index < lines.length) {
+    const prefix = lines[index][0]
+    if (prefix === " " || !prefix) {
+      rows++
+      index++
+      continue
+    }
+    if (prefix === "\\") {
+      index++
+      continue
+    }
+
+    let additions = 0
+    let deletions = 0
+    while (
+      index < lines.length &&
+      (lines[index][0] === "+" || lines[index][0] === "-")
+    ) {
+      if (lines[index][0] === "+") additions++
+      if (lines[index][0] === "-") deletions++
+      index++
+    }
+    rows += Math.max(additions, deletions)
+  }
+
+  return rows
+}