fix: background-colour erase (bce) for ESC[K past written text

lib/src/core/buffer/line.dart

@@ -145,6 +145,22 @@ class BufferLine with IndexedItem {
       eraseCell(end - 1, style);
     }
 
+    // Background-colour erase (bce): a line is only painted up to `_length`, so clamping the erase to
+    // `_length` would silently drop cells an app expects filled with the current background — e.g. codex
+    // sets a diff row's background then emits `ESC[K` to colour the rest of the row, leaving the row
+    // patchy instead of full-width. When the pen carries an explicit background, grow the line to the
+    // erase boundary so those cells exist and render. (Default-background erases stay trimmed, preserving
+    // the previous behaviour and keeping trailing cells out of selection/copy.)
+    if (style.background != 0 && end > _length) {
+      final gapStart = _length;
+      resize(end);
+      // A cursor parked past the written text leaves a gap before `start`; keep it default rather than
+      // whatever stale data the grown buffer held.
+      for (var i = gapStart; i < start && i < end; i++) {
+        resetCell(i);
+      }
+    }
+
     end = min(end, _length);
     for (var i = start; i < end; i++) {
       eraseCell(i, style);

test/src/core/buffer/bce_test.dart

@@ -0,0 +1,33 @@
+import 'package:test/test.dart';
+import 'package:xterm/core.dart';
+
+void main() {
+  group('background-colour erase (bce)', () {
+    test('ESC[K after short text fills the rest of the row with the current background', () {
+      final terminal = Terminal();
+      terminal.resize(20, 5);
+
+      // Green background, two characters, then erase-to-end-of-line.
+      terminal.write('\x1b[42mAB\x1b[K');
+
+      final line = terminal.buffer.lines[0];
+      final green = line.getBackground(0); // the background used for the written cells
+      expect(green, isNot(0), reason: 'sanity: an explicit background is set');
+
+      // Cells past the written text must carry the same background (bce), not the default.
+      expect(line.getBackground(2), green);
+      expect(line.getBackground(10), green);
+      expect(line.getBackground(19), green);
+    });
+
+    test('ESC[K with the default background does not extend the line', () {
+      final terminal = Terminal();
+      terminal.resize(20, 5);
+
+      terminal.write('AB\x1b[K');
+
+      // No explicit background → erased cells stay at the default (0); the line is not grown.
+      expect(terminal.buffer.lines[0].getBackground(10), 0);
+    });
+  });
+}