bug#39967: 27.0.90; how to debug point moving erratically?

[William Rankin <william@bydasein.com>]

> On 7 Mar 2020, at 11:06 pm, Eli Zaretskii <eliz@gnu.org> wrote:
> 
> If this happens when Emacs is idle, the first suspect is some idle
> timer you have running.  The variable timer-idle-list holds the list
> of the active idle timers, so look there for some hints.

This is what made me suspect jit-lock, because of the 5 sec idle timer 
followed by jumping at 0.5 secs intervals.

Here's my timer list:

           0.37     0.01 blink-cursor-timer-function
           6.59        - undo-auto--boundary-timer
          47.03     1.00 display-time-event-handler
   *       0.00        t show-paren-function
   *       0.50        t #f(compiled-function () #<bytecode 0x41903951> [jit-lock--antiblink-grace-timer jit-lock-context-fontify])
   *       0.50  :repeat blink-cursor-start
   *       1.00        t which-key--update
   *       5.00        t jit-lock-stealth-fontify

Could the sporadic nature hint at the byte-compiled function that calls 
jit-lock-context-fontify? (Also I see jit-lock-antiblink-grace is new 
in 27.)

Of course there's also which-key, so I've turned that off for the time
being.