Fix #3789: fix eglot lookup-documentation handler

Eglot replaced `eglot-help-at-point' in joaotavora/eglot@a044dec,
breaking our documentation lookup handler.

modules/tools/lsp/autoload/eglot.el

@@ -9,11 +9,31 @@ Example : (set-eglot-client! 'python-mode `(,(concat doom-etc-dir \"lsp/mspyls/M
   (after! eglot
     (add-to-list 'eglot-server-programs `(,mode . ,server-call))))
 
+;; HACK Eglot removed `eglot-help-at-point' in joaotavora/eglot@a044dec for a
+;;      more problematic approach of deferred to eldoc. Here, I've restored it.
+;;      Doom's lookup handlers try to open documentation in a separate window
+;;      (so they can be copied or kept open), but doing so with an eldoc buffer
+;;      is difficult because a) its contents are generated asynchronously,
+;;      making them tough to scrape, and b) their contents change frequently
+;;      (every time you move your cursor).
+(defvar +eglot--help-buffer nil)
 ;;;###autoload
-(defun +eglot/documentation-lookup-handler ()
-  "Documentation lookup handler using eglot :document/hover handler.
-
-Mostly a rewrite of `eglot-help-at-point', which should be used interactively."
-  (interactive)
-  (eglot-help-at-point)
-  (display-buffer eglot--help-buffer))
+(defun +eglot-lookup-documentation (_identifier)
+  "Request documentation for the thing at point."
+  (eglot--dbind ((Hover) contents range)
+      (jsonrpc-request (eglot--current-server-or-lose) :textDocument/hover
+                       (eglot--TextDocumentPositionParams))
+    (let ((blurb (and (not (seq-empty-p contents))
+                      (eglot--hover-info contents range)))
+          (hint (thing-at-point 'symbol)))
+      (if blurb
+          (with-current-buffer
+              (or (and (buffer-live-p +eglot--help-buffer)
+                       +eglot--help-buffer)
+                  (setq +eglot--help-buffer (generate-new-buffer "*eglot-help*")))
+            (with-help-window (current-buffer)
+              (rename-buffer (format "*eglot-help for %s*" hint))
+              (with-current-buffer standard-output (insert blurb))
+              (setq-local nobreak-char-display nil)))
+        (display-local-help))))
+  'deferred)