Thanks for implementing this. I have been testing it and it works pretty well.
  - I would add  this.add_hook("move_buffer_hook") in buffer_count_widget (mode-line.js) to update the
„[13/13]“ part in the modeline.
  - Each reordering should be written to the session file. Now, if I change the order of a tab and close
Conkeror without adding/removing other tabs, the original ordering is saved. So session_auto_save_save
should be run in move_buffer_hook too
  - It doesn't work when the tab bar is disabled, due to move_buffer_hook always calling
tab_bar_move_buffer. I suppose it could rearrange buffers so that you'll notice those invisible changes
when you turn the tab bar on again.