* The 2D matrix widget cannot make a single row read-only, so the time off row looks
  editable even though any attempt to change it is rejected. Making the cell read-only in
  the client would require an extension of ``web_widget_x2many_2d_matrix``.
* Lines generated from public holidays (``global_leave_id``) are not covered: they are
  not protected by ``project_timesheet_holidays`` and therefore never blocked the sheet.