Select and copy diff captures line numbers improperly

[davidfstr]

See failing test stubs:

# === Test: Select and Copy ===

@pytest.mark.skip('fails: currently also selects line numbers')
def test_can_click_and_drag_within_diff_text_to_select_diff_text() -> None:
    # ...including + and - markers but excluding line numbers
    pass


@pytest.mark.skip('fails: currently also copies line numbers')
def test_given_diff_text_selected_when_press_command_c_then_copies_selected_text() -> None:
    # ...including + and - markers but excluding line numbers
    pass