When an error occurs in Lisp code which redisplay has invoked, Emacs’s usual debugging mechanisms are unusable, for technical reasons. This subsection describes how to get a backtrace from such an error, which should be helpful in debugging it.
These directions apply to Lisp forms used, for example, in
:eval mode line constructs (see The Data Structure of the Mode Line), and in all
hooks invoked from redisplay, such as:
fontification-functions (see Automatic Face Assignment).
window-scroll-functions (see Hooks for Window Scrolling and Changes).
Note that if you have had an error in a hook function called from
redisplay, the error handling might have removed this function from
the hook. You will thus need to reinitialize that hook somehow,
perhaps with add-hook, to be able to replay the bug.
To generate a backtrace in these circumstances, set the variable
backtrace-on-redisplay-error to non-nil. When the error
occurs, Emacs will dump the backtrace to the buffer
*Redisplay-trace*, but won’t automatically display it in a
window. This is to avoid needlessly corrupting the redisplay you are
debugging. You will thus need to display the buffer yourself, with a
command such as switch-to-buffer-other-frame C-x 5 b.
Set this variable to non-nil to enable the generation of a
backtrace when an error occurs in any Lisp called from redisplay.