When checking code, IDLE changed focus to Shell in case there are warning messages. (I want to change this, but...) If there is a syntax error, current code calls messagebox.showerror and tries to put focus on the editor. The latter raised the editor over Shell but focus remains on the modal messagebox until it is dismissed. Error box is centered. After I put the focus line first, nothing changes. Should it? Is not a visible focused parent 'mapped'? idlelib.runscript 206/7.

        self.editwin.text.focus_set()
        tkMessageBox.showerror(title, message, parent=self.editwin.text)

Edit: After reading the patch and tkinter.messagebox, the error box should not be changed as it is a commondialog rather than simple dialog. Do you plan to change the commondialogs also? To me, they need it more.

The only easily trigger simpledialog in IDLE is askinteger('Tab width') for Format => Tabify and Untabify. If I select either of those with a mouse and want to click [OK] to accept the default of 4, I actually prefer the box where it is, up near the menu. Either way, entering a value out of the range puts an error messagebox in the center of the screen.

Edit2: IDLE's query.Query boxes put error messages in red in the entry box. I think that this would be an improvement for askinteger.

I have read the code but not checked it in detail. It does center 'Tab width' and I consider this a better default position even if not the best for this particular box. The 5 types of query.Query boxes are centered and I am satisfied with that and have never considered moving any of them.

Dialogs in commondialog are implemented in Tk and may use native OS dialogs. They are out of control Tkinter and may be out of control of Tk. Dialogs in simpledialog are implemented in Python and try to mimic dialogs in commondialog (more or less).

This PR just adds the code used to place native Tk dialogs rewritten to Python.

Moved the code to separate function (which corresponds Tk private function :tk:PlaceWindow) and resused it in SimpleDialog. There was similar code, but the new code better handles corner cases and supports virtual root window.

It can be reused also in IDLE, but I left this to separate PR.

If I decide to use _place_window before 3.8 or 3.9 leave maintenance, I will put a copy in idlelib for those versions.