Marker leak and undo history size explosion

[CyberShadow]

Steps to reproduce:

1. Run:

   emacs -Q \
     -l ~/.emacs.d/straight/repos/rich-minority/rich-minority.el \
     -l ~/.emacs.d/straight/repos/smart-mode-line/smart-mode-line.el \
     --eval '(setq custom-safe-themes t)' --eval '(sml/setup)'

   (adjust paths accordingly)

2. Type something (about 20-50 characters), then undo it.

3. Repeat step above 2-3 times.

4. Evaluate (press M-:, paste, then RET):

   (let ((n 0))
    (mapc (lambda (el)
        (when (and (consp el) (markerp (car el)))
          (setq n (1+ n))))
      buffer-undo-list)
    n))

Normally (without SML loaded), this produces a small number. However, with SML loaded, the number is very large - in the hundreds / thousands.

SML or something it uses is creating markers in the buffer, which end up being referenced in the undo history. The undo history length grows exponentially.

I suspect that this is [partially] responsible for the number of times one can undo being very small in practice.

Possibly related Emacs bug:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=52798