>>> > +(defvar tab-line-tabs-window-buffers-filter-function
>>> > +  #'tab-line-tabs-window-buffers-filter-default
>>> > +  "Filter which buffers should be displayed in the tab line.")
>>> 
>>> Shouldn't this be defcustom?  Then you can add a const 'identity'
>>> to allow keeping excluded buffers.
>>
>> Didn't you say that keeping them produces buggy behavior?  If so, why
>> would we tempt users to produce bugs?
>
> It's not so simple.  Currently tab-line is disabled on these conditions:
>
>               (string-match-p "\\` " (buffer-name))
>               (memq major-mode tab-line-exclude-modes)
>               (buffer-match-p tab-line-exclude-buffers (buffer-name))
>               (get major-mode 'tab-line-exclude)
>               (buffer-local-value 'tab-line-exclude (current-buffer)))
>
> Buggy behavior is produced only on the first condition:
> when the buffer name begins with a space.
>
> So the default predicate should use exactly the same conditions
> in the filter to hide buffers on the tab-line.
>
> And also defcustom should provide another value to get back
> almost the same behavior as current, with one exception that
> buffers with names that begin with a space should be hidden too,
> i.e. the second value should match only one condition:
>
>               (string-match-p "\\` " (buffer-name))

From what I have tested, buffers that start with a space are unlikely to
appear on the tab line by default, so i don't think a new value is
necessary.

-- 
- E.G via Gnus and Org.