Wait for visibility before setting a grab. Fixes #946153.
diff --git a/Lib/lib-tk/tkSimpleDialog.py b/Lib/lib-tk/tkSimpleDialog.py
index 01709d8..4d11ce0 100644
--- a/Lib/lib-tk/tkSimpleDialog.py
+++ b/Lib/lib-tk/tkSimpleDialog.py
@@ -61,6 +61,7 @@
 
         self.buttonbox()
 
+        self.wait_visibility() # window needs to be visible for the grab
         self.grab_set()
 
         if not self.initial_focus: