fit lsp-ui-imenu to longest line in buffer (#764)

Previously, in buffers with many nested levels of imenu, the long lines
would trail off past the width of the window. Now, we properly enlarge
the window to fit the longest line.

lsp-ui-imenu.el

@@ -323,12 +323,22 @@ ITEMS are used when the kind position is `left."
 	  (let ((window-size-fixed)) ;; Temporarily set `window-size-fixed' to nil for resizing.
 	;; When `lsp-ui-imenu-window-width' is 0, fit window to buffer:
 	(if (= lsp-ui-imenu-window-width 0)
-			(let ((fit-window-to-buffer-horizontally 'only))
-			  (fit-window-to-buffer win)
-			  (window-resize win 3 t))
-		  (let ((x (- lsp-ui-imenu-window-width (window-width))))
-			(window-resize (selected-window) x t))))
-	  )))
+            (let ((actual-width (if (fboundp 'buffer-line-statistics)
+                                    ;; since Emacs-28
+                                    (cadr (buffer-line-statistics))
+                                  (save-excursion
+                                    (goto-char (point-min))
+                                    (let ((max 0)
+                                          (to (point-max)))
+                                      (while (< (point) to)
+                                        (end-of-line)
+                                        (setq max (max max (current-column)))
+                                        (forward-line))
+                                      max)))))
+              (enlarge-window-horizontally
+               (- (1+ actual-width) (window-width win))))
+          (let ((x (- lsp-ui-imenu-window-width (window-width))))
+            (window-resize (selected-window) x t)))))))
 
 (defun lsp-ui-imenu--kill nil
   "Kill imenu window."