From 3a7c18bc52f87d23284a34a942cfd78460f2763c Mon Sep 17 00:00:00 2001
From: Hannes Rudolph <hrudolph@gmail.com>
Date: Tue, 11 Nov 2025 12:56:30 -0700
Subject: [PATCH] webview-ui: fix chat diff mis-render for EOF append by
 collapsing identical -/+ pairs into context; add unit tests

---
 .../utils/__tests__/parseUnifiedDiff.spec.ts  | 31 ++++++++++++++++++
 webview-ui/src/utils/parseUnifiedDiff.ts      | 32 ++++++++++++++++++-
 2 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 webview-ui/src/utils/__tests__/parseUnifiedDiff.spec.ts

diff --git a/webview-ui/src/utils/__tests__/parseUnifiedDiff.spec.ts b/webview-ui/src/utils/__tests__/parseUnifiedDiff.spec.ts
new file mode 100644
index 0000000000..a465891b11
--- /dev/null
+++ b/webview-ui/src/utils/__tests__/parseUnifiedDiff.spec.ts
@@ -0,0 +1,31 @@
+import { parseUnifiedDiff } from "@/utils/parseUnifiedDiff"
+
+describe("parseUnifiedDiff - collapse identical -/+ pairs", () => {
+	it("collapses deletion+addition of identical text into a single context line", () => {
+		// Typical trailing-newline-only change at EOF with an additional appended line
+		const diff = ["@@ -1,1 +1,2 @@", "-old", "+old", "+new", ""].join("\n")
+
+		const lines = parseUnifiedDiff(diff)
+
+		// Should normalize the replace of identical line into context, plus the appended line
+		expect(lines.map((l) => l.type)).toEqual(["context", "addition"])
+		expect(lines[0].content).toBe("old")
+		expect(lines[1].content).toBe("new")
+		// Line numbers should be preserved appropriately
+		expect(lines[0].oldLineNum).toBe(1)
+		expect(lines[0].newLineNum).toBe(1)
+		expect(lines[1].oldLineNum).toBeNull()
+		expect(lines[1].newLineNum).toBe(2)
+	})
+
+	it("does not collapse when content differs (true replacement)", () => {
+		const diff = ["@@ -1,1 +1,1 @@", "-old", "+new", ""].join("\n")
+
+		const lines = parseUnifiedDiff(diff)
+
+		// Keep as deletion + addition for a real replacement
+		expect(lines.map((l) => l.type)).toEqual(["deletion", "addition"])
+		expect(lines[0].content).toBe("old")
+		expect(lines[1].content).toBe("new")
+	})
+})
diff --git a/webview-ui/src/utils/parseUnifiedDiff.ts b/webview-ui/src/utils/parseUnifiedDiff.ts
index bed84c4ca9..72bef6bb91 100644
--- a/webview-ui/src/utils/parseUnifiedDiff.ts
+++ b/webview-ui/src/utils/parseUnifiedDiff.ts
@@ -88,7 +88,37 @@ export function parseUnifiedDiff(source: string, filePath?: string): DiffLine[]
 			prevHunk = hunk
 		}
 
-		return lines
+		// Collapse "- line" then "+ same line" pairs into a single context line.
+		// This normalizes diffs where the only change is adding a trailing newline
+		// (common when appending to a file missing EOF newline). VS Code's diff
+		// shows these as unchanged; our chat view should too.
+		const collapseReplacePairs = (input: DiffLine[]): DiffLine[] => {
+			const out: DiffLine[] = []
+			for (let i = 0; i < input.length; i++) {
+				const cur = input[i]
+				const next = input[i + 1]
+				if (
+					cur &&
+					next &&
+					cur.type === "deletion" &&
+					next.type === "addition" &&
+					cur.content === next.content
+				) {
+					out.push({
+						oldLineNum: cur.oldLineNum,
+						newLineNum: next.newLineNum,
+						type: "context",
+						content: cur.content,
+					})
+					i++ // skip the paired addition
+					continue
+				}
+				out.push(cur)
+			}
+			return out
+		}
+
+		return collapseReplacePairs(lines)
 	} catch {
 		// swallow parse errors and render nothing rather than breaking the UI
 		return []