flycheck: use pos-tip in GUI Emacs (linux), popup-tip everywhere else

Long story short, `flycheck-popup-tip' works everywhere but only looks
*ok*.  `flycheck-pos-tip' looks great, but only in GUI Emacs on Linux.
So we want:

+ GUI Emacs (Linux): pos-tip
+ GUI Emacs (MacOS): popup-tip
+ tty Emacs (anywhere): popup-tip

modules/feature/syntax-checker/config.el

@@ -1,37 +1,44 @@
 ;;; feature/syntax-checker/config.el -*- lexical-binding: t; -*-
 
-;; pkg-info doesn't get autoloaded when `flycheck-version' needs it, so we do
-;; it ourselves:
-(autoload 'pkg-info-version-info "pkg-info")
-
 (def-package! flycheck
   :commands (flycheck-mode flycheck-list-errors flycheck-buffer)
   :config
-  ;; Emacs feels snappier without checks on idle/change
-  (setq flycheck-check-syntax-automatically '(save mode-enabled))
-
-  (set! :popup 'flycheck-error-list-mode :select t :autokill t)
+  ;; Emacs feels snappier without checks on newline
+  (setq flycheck-check-syntax-automatically '(save idle-change mode-enabled))
 
   (after! evil
-    ;; Flycheck buffer on ESC in normal mode.
     (defun +syntax-checkers|flycheck-buffer ()
+      "Flycheck buffer on ESC in normal mode."
       (when flycheck-mode
         (ignore-errors (flycheck-buffer))
         nil))
-    (add-hook '+evil-esc-hook #'+syntax-checkers|flycheck-buffer t)))
+    (add-hook '+evil-esc-hook #'+syntax-checkers|flycheck-buffer t)
 
+    ;; With the option of flychecking the buffer on escape, so we don't need
+    ;; auto-flychecking on idle-change:
+    (delq 'idle-change flycheck-check-syntax-automatically)))
+
+
+;; Long story short, `flycheck-popup-tip' works everywhere but only looks *ok*.
+;; `flycheck-pos-tip' looks great, but only in GUI Emacs on Linux. So we want:
+;;
+;; + GUI Emacs (Linux): pos-tip
+;; + GUI Emacs (MacOS): popup-tip
+;; + tty Emacs (anywhere): popup-tip
 
 (def-package! flycheck-pos-tip
-  :unless IS-MAC
-  :after flycheck
+  :commands (flycheck-pos-tip-mode)
   :config
   (setq flycheck-pos-tip-timeout 10
-        flycheck-display-errors-delay 0.5)
-  (flycheck-pos-tip-mode +1))
-
+        ;; fallback to flycheck-popup-tip in terminal Emacs
+        flycheck-pos-tip-display-errors-tty-function
+        #'flycheck-popup-tip-show-popup
+        flycheck-display-errors-delay 0.5))
 
 (def-package! flycheck-popup-tip
-  :when IS-MAC
-  :after flycheck
-  :hook (flycheck-mode . flycheck-popup-tip-mode))
+  :commands (flycheck-popup-tip-mode flycheck-popup-tip-show-popup))
 
+(after! flycheck
+  (if IS-MAC
+      (flycheck-popup-tip)
+    (flycheck-pos-tip-mode)))