gh-90304: Fix IDLE startup failure with a broken font

A font reporting a zero width for '0' made set_width() raise
ZeroDivisionError. It now falls back to the configured width.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/idlelib/editor.py

@@ -329,7 +329,9 @@ def set_width(self):
         # http://www.tcl.tk/man/tcl8.6/TkCmd/text.htm#M21
         zero_char_width = \
             Font(text, font=text.cget('font')).measure('0')
-        self.width = pixel_width // zero_char_width
+        # Some fonts report a zero width for '0' (gh-90304).
+        self.width = (pixel_width // zero_char_width if zero_char_width
+                      else text.tk.getint(text.cget('width')))
 
     def new_callback(self, event):
         dirname, basename = self.io.defaultfilename()

Lib/idlelib/idle_test/test_editor.py

@@ -3,6 +3,7 @@
 from idlelib import editor
 import unittest
 from collections import namedtuple
+from unittest import mock
 from test.support import requires
 from tkinter import Tk, Text
 
@@ -30,6 +31,18 @@ def test_init(self):
         self.assertEqual(e.root, self.root)
         e._close()
 
+    def test_set_width_zero_char_width(self):
+        # A zero-width '0' must not raise ZeroDivisionError (gh-90304).
+        e = Editor(root=self.root)
+        try:
+            with mock.patch.object(editor, 'Font') as MockFont:
+                MockFont.return_value.measure.return_value = 0
+                e.set_width()
+            self.assertEqual(e.width,
+                             e.text.tk.getint(e.text.cget('width')))
+        finally:
+            e._close()
+
 
 class GetLineIndentTest(unittest.TestCase):
     def test_empty_lines(self):

Misc/NEWS.d/next/IDLE/2026-06-27-17-27-21.gh-issue-90304.464d22.rst

@@ -0,0 +1,4 @@
+Prevent IDLE from failing to start when a font reports a zero width for
+the ``'0'`` character.  Such a broken font caused a
+:exc:`ZeroDivisionError`; the editor now falls back to the configured
+width.