fix(ui): render whole-file patches as complete diffs (#30516)

packages/ui/src/components/session-diff.ts

@@ -60,13 +60,68 @@ function fileDiffFromPatch(file: string, patch: string) {
     return hit
   }
 
-  const input = patchInput(file, patch)
-  const value = (input ? parsePatchFiles(input)[0]?.files[0] : undefined) ?? emptyFileDiff(file)
+  const contents = completePatchContents(patch)
+  const input = contents ? undefined : patchInput(file, patch)
+  const value = contents
+    ? fileDiffFromContent(file, contents.before, contents.after)
+    : (input ? parsePatchFiles(input)[0]?.files[0] : undefined) ?? emptyFileDiff(file)
   patchFileDiffCache.set(key, value)
   while (patchFileDiffCache.size > diffCacheLimit) patchFileDiffCache.delete(patchFileDiffCache.keys().next().value!)
   return value
 }
 
+function completePatchContents(patch: string) {
+  try {
+    const parsed = parsePatch(patch)[0]
+    if (!parsed || (!parsed.index && !parsed.oldFileName && !parsed.newFileName)) return
+    // Snapshot and VCS producers request full context. Tool patches use jsdiff's shorter default context.
+    if (!patch.startsWith("diff --git ") && !/^--- [^\n]*\t\r?\n\+\+\+ [^\n]*\t(?:\r?\n|$)/m.test(patch)) return
+    // Full patches collapse into one leading hunk. Separated hunks omit ranges and must stay partial.
+    if (parsed.hunks.length !== 1) return
+
+    const hunk = parsed.hunks[0]
+    if (!hunk || hunk.oldStart > 1 || hunk.newStart > 1) return
+
+    const before: Array<{ text: string; newline: boolean }> = []
+    const after: Array<{ text: string; newline: boolean }> = []
+    let previous: "-" | "+" | " " | undefined
+
+    for (const line of hunk.lines) {
+      if (line.startsWith("\\")) {
+        if (previous === "-" || previous === " ") {
+          const value = before.at(-1)
+          if (value) value.newline = false
+        }
+        if (previous === "+" || previous === " ") {
+          const value = after.at(-1)
+          if (value) value.newline = false
+        }
+        continue
+      }
+      if (line.startsWith("-")) {
+        before.push({ text: line.slice(1), newline: true })
+        previous = "-"
+        continue
+      }
+      if (line.startsWith("+")) {
+        after.push({ text: line.slice(1), newline: true })
+        previous = "+"
+        continue
+      }
+      if (!line.startsWith(" ")) return
+      before.push({ text: line.slice(1), newline: true })
+      after.push({ text: line.slice(1), newline: true })
+      previous = " "
+    }
+
+    const text = (lines: Array<{ text: string; newline: boolean }>) =>
+      lines.map((line) => line.text + (line.newline ? "\n" : "")).join("")
+    return { before: text(before), after: text(after) }
+  } catch {
+    return
+  }
+}
+
 function patchInput(file: string, patch: string) {
   try {
     const parsed = parsePatch(patch)[0]