bpo-42685: Improve placing of simple query windows. (GH-23856)

* If parent is specified and mapped, the query widget is
  centered at the center of parent. Its position and size
  can be corrected so that it fits in the virtual root window.
* Otherwise it is centered at the center of the screen.

Lib/tkinter/simpledialog.py

@@ -55,36 +55,8 @@ class SimpleDialog:
                 b.config(relief=RIDGE, borderwidth=8)
             b.pack(side=LEFT, fill=BOTH, expand=1)
         self.root.protocol('WM_DELETE_WINDOW', self.wm_delete_window)
-        self._set_transient(master)
-
-    def _set_transient(self, master, relx=0.5, rely=0.3):
-        widget = self.root
-        widget.withdraw() # Remain invisible while we figure out the geometry
-        widget.transient(master)
-        widget.update_idletasks() # Actualize geometry information
-        if master.winfo_ismapped():
-            m_width = master.winfo_width()
-            m_height = master.winfo_height()
-            m_x = master.winfo_rootx()
-            m_y = master.winfo_rooty()
-        else:
-            m_width = master.winfo_screenwidth()
-            m_height = master.winfo_screenheight()
-            m_x = m_y = 0
-        w_width = widget.winfo_reqwidth()
-        w_height = widget.winfo_reqheight()
-        x = m_x + (m_width - w_width) * relx
-        y = m_y + (m_height - w_height) * rely
-        if x+w_width > master.winfo_screenwidth():
-            x = master.winfo_screenwidth() - w_width
-        elif x < 0:
-            x = 0
-        if y+w_height > master.winfo_screenheight():
-            y = master.winfo_screenheight() - w_height
-        elif y < 0:
-            y = 0
-        widget.geometry("+%d+%d" % (x, y))
-        widget.deiconify() # Become visible at the desired location
+        self.root.transient(master)
+        _place_window(self.root, master)
 
     def go(self):
         self.root.wait_visibility()
@@ -157,11 +129,7 @@ class Dialog(Toplevel):
 
         self.protocol("WM_DELETE_WINDOW", self.cancel)
 
-        if parent is not None:
-            self.geometry("+%d+%d" % (parent.winfo_rootx()+50,
-                                      parent.winfo_rooty()+50))
-
-        self.deiconify() # become visible now
+        _place_window(self, parent)
 
         self.initial_focus.focus_set()
 
@@ -251,6 +219,37 @@ class Dialog(Toplevel):
         pass # override
 
 
+# Place a toplevel window at the center of parent or screen
+# It is a Python implementation of ::tk::PlaceWindow.
+def _place_window(w, parent=None):
+    w.wm_withdraw() # Remain invisible while we figure out the geometry
+    w.update_idletasks() # Actualize geometry information
+
+    minwidth = w.winfo_reqwidth()
+    minheight = w.winfo_reqheight()
+    maxwidth = w.winfo_vrootwidth()
+    maxheight = w.winfo_vrootheight()
+    if parent is not None and parent.winfo_ismapped():
+        x = parent.winfo_rootx() + (parent.winfo_width() - minwidth) // 2
+        y = parent.winfo_rooty() + (parent.winfo_height() - minheight) // 2
+        vrootx = w.winfo_vrootx()
+        vrooty = w.winfo_vrooty()
+        x = min(x, vrootx + maxwidth - minwidth)
+        x = max(x, vrootx)
+        y = min(y, vrooty + maxheight - minheight)
+        y = max(y, vrooty)
+        if w._windowingsystem == 'aqua':
+            # Avoid the native menu bar which sits on top of everything.
+            y = max(y, 22)
+    else:
+        x = (w.winfo_screenwidth() - minwidth) // 2
+        y = (w.winfo_screenheight() - minheight) // 2
+
+    w.wm_maxsize(maxwidth, maxheight)
+    w.wm_geometry('+%d+%d' % (x, y))
+    w.wm_deiconify() # Become visible at the desired location
+
+
 # --------------------------------------------------------------------
 # convenience dialogues