Set buffers-tab-filter-functions to nil to display all open files as
tabs.
If you want to have tabs for files with the same mode (current
behavior) and all your Planner tabs, use the following snippet:
(defun paul/always-show-planner-buffers (buffer-to-select buf1)
"Show same-mode buffers and planner buffers."
(or (select-buffers-tab-buffers-by-mode buffer-to-select buf1)
(eq 'planner-mode (symbol-value-in-buffer 'major-mode buffer-to-select))))
(setq buffers-tab-filter-functions (list 'paul/always-show-planner-buffers))
Hey, this XEmacs thing isn't that bad...