Fix #13: Expose Pre-edit Backup restoration

Preserve full diff opcodes before grouping, route modal navigation to a two-axis viewport-sized scroller, and calculate side-by-side layout using terminal cell widths so tabbed Hosts File lines keep a stable divider.

tests/test_restore_preview.py

@@ -0,0 +1,73 @@
+"""Tests for the exact-file preview shown before Pre-edit Backup restoration."""
+
+import pytest
+
+from hosts.core.restore_preview import RestorePreviewError, create_restore_preview
+
+
+def test_preview_pairs_current_and_restored_lines_in_changed_hunks(tmp_path):
+    """The current Hosts File is the left/before side of the preview."""
+    backup = tmp_path / "hosts.backup"
+    hosts_file = tmp_path / "hosts"
+    backup.write_text("127.0.0.1 localhost\n192.0.2.1 restored.test\n")
+    hosts_file.write_text("127.0.0.1 localhost\n192.0.2.2 current.test\n")
+
+    preview = create_restore_preview(backup, hosts_file)
+
+    assert preview.has_changes
+    row = preview.changed_hunks[0].rows[1]
+    assert row.current_line_number == 2
+    assert row.restored_line_number == 2
+    assert row.current_text == "192.0.2.2 current.test"
+    assert row.restored_text == "192.0.2.1 restored.test"
+
+
+def test_preview_distinguishes_missing_final_newlines(tmp_path):
+    backup = tmp_path / "hosts.backup"
+    hosts_file = tmp_path / "hosts"
+    backup.write_bytes(b"127.0.0.1 localhost\n")
+    hosts_file.write_bytes(b"127.0.0.1 localhost")
+
+    preview = create_restore_preview(backup, hosts_file)
+
+    assert preview.has_changes
+    assert preview.restored_has_final_newline
+    assert not preview.current_has_final_newline
+
+
+def test_preview_rejects_files_that_cannot_be_decoded_as_hosts_text(tmp_path):
+    backup = tmp_path / "hosts.backup"
+    hosts_file = tmp_path / "hosts"
+    backup.write_bytes(b"\xff")
+    hosts_file.write_text("127.0.0.1 localhost\n")
+
+    with pytest.raises(RestorePreviewError, match="Pre-edit Backup"):
+        create_restore_preview(backup, hosts_file)
+
+
+def test_preview_detects_when_either_reviewed_file_changes(tmp_path):
+    backup = tmp_path / "hosts.backup"
+    hosts_file = tmp_path / "hosts"
+    backup.write_text("127.0.0.1 localhost\n")
+    hosts_file.write_text("192.0.2.2 current.test\n")
+    preview = create_restore_preview(backup, hosts_file)
+
+    hosts_file.write_text("192.0.2.3 changed.test\n")
+
+    assert not preview.matches_files()
+
+
+def test_full_preview_keeps_lines_outside_changed_hunk_context(tmp_path):
+    """Full mode retains the complete files after changed hunks are calculated."""
+    backup = tmp_path / "hosts.backup"
+    hosts_file = tmp_path / "hosts"
+    current_lines = [f"192.0.2.{index} host-{index}.test" for index in range(1, 41)]
+    restored_lines = current_lines.copy()
+    restored_lines[19] = "192.0.2.20 restored.test"
+    hosts_file.write_text("\n".join(current_lines) + "\n")
+    backup.write_text("\n".join(restored_lines) + "\n")
+
+    preview = create_restore_preview(backup, hosts_file)
+
+    assert len(preview.changed_hunks[0].rows) == 7
+    assert len(preview.full_hunks[0].rows) == 40