optimization if updating the window's mode line.
if (XFASTINT (w->last_modified) >= MODIFF
&& PT >= startp && !clip_changed
&& (just_this_one || XFASTINT (w->width) == FRAME_WIDTH (f))
+ /* If force-mode-line-update was called, really redisplay;
+ that's how redisplay is forced after e.g. changing
+ buffer-invisibility-spec. */
+ && ! NILP (w->update_mode_line)
/* Can't use this case if highlighting a region. */
&& !(!NILP (Vtransient_mark_mode) && !NILP (current_buffer->mark_active))
&& NILP (w->region_showing)