[lsp-ui-doc] Conditionaly address issue #21

If we move/resize the frame after making it visible, it makes a
flickering effect.
So we do it only on systems where the bug is present (os x)

lsp-ui-doc.el

@@ -352,6 +352,11 @@ SYMBOL."
          mode-line-format nil
          cursor-type nil)))
 
+(defun lsp-ui-doc--buggy-system-p ()
+  "Return non-nil if there is a bug with frames on the system.
+https://github.com/emacs-lsp/lsp-ui/issues/21"
+  (eq 'ns (window-system)))
+
 (defun lsp-ui-doc--display (symbol string)
   "Display the documentation on screen.
 SYMBOL STRING."
@@ -361,9 +366,12 @@ SYMBOL STRING."
     (lsp-ui-doc--render-buffer string symbol)
     (unless (frame-live-p (lsp-ui-doc--get-frame))
       (lsp-ui-doc--set-frame (lsp-ui-doc--make-frame)))
+    (unless (lsp-ui-doc--buggy-system-p)
+      (lsp-ui-doc--move-frame (lsp-ui-doc--get-frame)))
     (unless (frame-visible-p (lsp-ui-doc--get-frame))
       (make-frame-visible (lsp-ui-doc--get-frame)))
-    (lsp-ui-doc--move-frame (lsp-ui-doc--get-frame))))
+    (when (lsp-ui-doc--buggy-system-p)
+      (lsp-ui-doc--move-frame (lsp-ui-doc--get-frame)))))
 
 (defun lsp-ui-doc--make-frame ()
   "Create the child frame and return it."