From 3bb115a39c7c2c29266fc38e79ded2a186d9f7f4 Mon Sep 17 00:00:00 2001
From: Mario Zechner <badlogicgames@gmail.com>
Date: Sun, 11 Jan 2026 03:00:23 +0100
Subject: [PATCH] fix(tui): cursor position tracking when content shrinks with
 unchanged lines

---
 packages/tui/CHANGELOG.md            |  1 +
 packages/tui/src/tui.ts              | 25 ++++++++++++++++
 packages/tui/test/tui-render.test.ts | 43 ++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)
 create mode 100644 packages/tui/test/tui-render.test.ts

diff --git a/packages/tui/CHANGELOG.md b/packages/tui/CHANGELOG.md
index 1ce7dc71..a0860a0c 100644
--- a/packages/tui/CHANGELOG.md
+++ b/packages/tui/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Cursor position tracking when content shrinks with unchanged remaining lines
 - TUI renders with wrong dimensions after suspend/resume if terminal was resized while suspended ([#599](https://github.com/badlogic/pi-mono/issues/599))
 
 ## [0.42.4] - 2026-01-10
diff --git a/packages/tui/src/tui.ts b/packages/tui/src/tui.ts
index ca5c8efe..e36ef822 100644
--- a/packages/tui/src/tui.ts
+++ b/packages/tui/src/tui.ts
@@ -385,6 +385,31 @@ export class TUI extends Container {
 			return;
 		}
 
+		// All changes are in deleted lines (nothing to render, just clear)
+		if (firstChanged >= newLines.length) {
+			if (this.previousLines.length > newLines.length) {
+				let buffer = "\x1b[?2026h";
+				// Move to end of new content
+				const targetRow = newLines.length - 1;
+				const lineDiff = targetRow - this.cursorRow;
+				if (lineDiff > 0) buffer += `\x1b[${lineDiff}B`;
+				else if (lineDiff < 0) buffer += `\x1b[${-lineDiff}A`;
+				buffer += "\r";
+				// Clear extra lines
+				const extraLines = this.previousLines.length - newLines.length;
+				for (let i = 0; i < extraLines; i++) {
+					buffer += "\r\n\x1b[2K";
+				}
+				buffer += `\x1b[${extraLines}A`;
+				buffer += "\x1b[?2026l";
+				this.terminal.write(buffer);
+				this.cursorRow = newLines.length - 1;
+			}
+			this.previousLines = newLines;
+			this.previousWidth = width;
+			return;
+		}
+
 		// Check if firstChanged is outside the viewport
 		// cursorRow is the line where cursor is (0-indexed)
 		// Viewport shows lines from (cursorRow - height + 1) to cursorRow
diff --git a/packages/tui/test/tui-render.test.ts b/packages/tui/test/tui-render.test.ts
new file mode 100644
index 00000000..61c6d155
--- /dev/null
+++ b/packages/tui/test/tui-render.test.ts
@@ -0,0 +1,43 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import { type Component, TUI } from "../src/tui.js";
+import { VirtualTerminal } from "./virtual-terminal.js";
+
+class TestComponent implements Component {
+	lines: string[] = [];
+	render(_width: number): string[] {
+		return this.lines;
+	}
+	invalidate(): void {}
+}
+
+describe("TUI differential rendering", () => {
+	it("tracks cursor correctly when content shrinks with unchanged remaining lines", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		// Initial render: 5 identical lines
+		component.lines = ["Line 0", "Line 1", "Line 2", "Line 3", "Line 4"];
+		tui.start();
+		await terminal.flush();
+
+		// Shrink to 3 lines, all identical to before (no content changes in remaining lines)
+		component.lines = ["Line 0", "Line 1", "Line 2"];
+		tui.requestRender();
+		await terminal.flush();
+
+		// cursorRow should be 2 (last line of new content)
+		// Verify by doing another render with a change on line 1
+		component.lines = ["Line 0", "CHANGED", "Line 2"];
+		tui.requestRender();
+		await terminal.flush();
+
+		const viewport = terminal.getViewport();
+		// Line 1 should show "CHANGED", proving cursor tracking was correct
+		assert.ok(viewport[1]?.includes("CHANGED"), `Expected "CHANGED" on line 1, got: ${viewport[1]}`);
+
+		tui.stop();
+	});
+});