fix: scroll a partial DECSTBM region within itself, not below it

lib/src/core/buffer/buffer.dart

@@ -235,7 +235,12 @@ class Buffer {
   void index() {
     if (isInVerticalMargin) {
       if (_cursorY == _marginBottom) {
-        if (marginTop == 0 && !isAltBuffer) {
+        // Growing the scrollback (insert a line just below the last visible row, so the top line is
+        // preserved as history) is only correct for a *full-height* scroll — a region whose bottom is
+        // the last row of the screen. codex/ratatui set *partial* scroll regions (e.g. `ESC[1;9r`,
+        // `ESC[10;35r`) to redraw part of its inline UI; scrolling one of those must shift lines *within*
+        // the region (`scrollUp`), not shove everything below it down — doing so scrambled the display.
+        if (marginTop == 0 && _marginBottom == viewHeight - 1 && !isAltBuffer) {
           lines.insert(absoluteMarginBottom + 1, _newEmptyLine());
         } else {
           scrollUp(1);

test/src/core/buffer/scroll_region_test.dart

@@ -0,0 +1,33 @@
+import 'package:test/test.dart';
+import 'package:xterm/core.dart';
+
+void main() {
+  group('partial DECSTBM scroll region', () {
+    test('scrolling a partial region does not disturb rows below it', () {
+      final terminal = Terminal();
+      terminal.resize(10, 6);
+
+      // Fill six rows with identifiable content.
+      terminal.write('L0\r\nL1\r\nL2\r\nL3\r\nL4\r\nL5');
+
+      // A *partial* scroll region covering rows 1..3 (indices 0..2); the screen is 6 rows tall.
+      terminal.write('\x1b[1;3r');
+      // Park the cursor on the bottom margin (row 3) and index — scrolls the region up by one.
+      terminal.write('\x1b[3;1H');
+      terminal.write('\n');
+
+      String row(int i) => terminal.buffer.lines[i].toString().trim();
+
+      // Rows below the region must be untouched — the bug inserted a line below the region and shoved
+      // these down.
+      expect(row(3), 'L3');
+      expect(row(4), 'L4');
+      expect(row(5), 'L5');
+
+      // The region itself scrolled up: L0 fell off the top, L1/L2 moved up, the bottom row cleared.
+      expect(row(0), 'L1');
+      expect(row(1), 'L2');
+      expect(row(2), '');
+    });
+  });
+}