jit-lock-debug-mode

jit-lock-debug-mode is a variable defined in `jit-lock.el'.
Its value is

nil

Documentation:
Non-nil if Jit-Lock-Debug mode is enabled.
See the command `jit-lock-debug-mode' for a description of this minor mode.
Setting this variable directly does not take effect;
either customize it (see the info node `Easy Customization')
or call the function `jit-lock-debug-mode'.

You can customize this variable.