bug#67463: 30.0.50; Eglot may manage js-json-mode buffers with wrong server

[Stefan Monnier]

>> So should we now close this bug?  Or is there something left to do
>> here?
> The problem is not solved and the workaround is tedious. There is something
> to be fixed, but outside Eglot.

Agreed.  I can see two solutions:

- Add a tool like `derived-mode-remove-parent`.
- Fix the definition of `js-json-mode` so it doesn't appear as a child
  of `js-mode`.

I think the second solution is best.
A quick&dirty way to do that could be the patch below.


        Stefan


diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index 0115feb0e97..ac99c96166c 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -3924,7 +3924,8 @@ js-ts--syntax-propertize
         (put-text-property (1- ne) ne 'syntax-table syntax)))))
 
 ;;;###autoload
-(define-derived-mode js-json-mode js-mode "JSON"
+(define-derived-mode js-json-mode prog-mode "JSON"
+  (js-mode) ;; For expediency, reuse js-mode, but not as parent (bug#67463).
   (setq-local js-enabled-frameworks nil)
   ;; Speed up `syntax-ppss': JSON files can be big but can't hold
   ;; regexp matchers nor #! thingies (and `js-enabled-frameworks' is nil).