Created attachment 246169 [details] [review]
A patch that fixes the problem

When doing updates, gtk_progress_bar_real_update can calculate
compute a negative value for activity_pos:

              if (pbar->activity_dir == 0)
                {
                  pbar->activity_pos += pbar->activity_step;
                  if (pbar->activity_pos + size >=
                      widget->allocation.width -
                      widget->style->xthickness)
                    {
                      pbar->activity_pos = widget->allocation.width -
                        widget->style->xthickness - size;
                      pbar->activity_dir = 1;
                    }
                }

If the allocation width is smaller than (xthickness + size),
pbar->activity_pos will be set to a negative value and this will
cause the drawing code to emit a warning about coordinates being
out of bounds (in this case x will be negative).

The fix is to make activity_pos always positive, like the attached
patch does.