** If you encounter X protocol errors

Try evaluating (x-synchronize t).  That puts Emacs into synchronous
mode, where each Xlib call checks for errors before it returns.  This
mode is much slower, but when you get an error, you will see exactly
which call really caused the error.

You can start Emacs in a synchronous mode by invoking it with the -xrm
option, like this:

    emacs -xrm "emacs.synchronous: true"

Setting a breakpoint in the function `x_error_quitter' and looking at
the backtrace when Emacs stops inside that function will show what
code causes the X protocol errors.

Some bugs related to the X protocol disappear when Emacs runs in a
synchronous mode.  To track down those bugs, we suggest the following
procedure:

  - Run Emacs under a debugger and put a breakpoint inside the
    primitive function which, when called from Lisp, triggers the X
    protocol errors.  For example, if the errors happen when you
    delete a frame, put a breakpoint inside `Fdelete_frame'.

  - When the breakpoint breaks, step through the code, looking for
    calls to X functions (the ones whose names begin with "X" or
    "Xt" or "Xm").

  - Insert calls to `XSync' before and after each call to the X
    functions, like this:

       XSync (f->output_data.x->display_info->display, 0);

    where `f' is the pointer to the `struct frame' of the selected
    frame, normally available via XFRAME (selected_frame).  (Most
    functions which call X already have some variable that holds the
    pointer to the frame, perhaps called `f' or `sf', so you shouldn't
    need to compute it.)

    If your debugger can call functions in the program being debugged,
    you should be able to issue the calls to `XSync' without recompiling
    Emacs.  For example, with GDB, just type:

       call XSync (f->output_data.x->display_info->display, 0)

    before and immediately after the suspect X calls.  If your
    debugger does not support this, you will need to add these pairs
    of calls in the source and rebuild Emacs.

    Either way, systematically step through the code and issue these
    calls until you find the first X function called by Emacs after
    which a call to `XSync' winds up in the function
    `x_error_quitter'.  The first X function call for which this
    happens is the one that generated the X protocol error.

  - You should now look around this offending X call and try to figure
    out what is wrong with it.