From 3ae24bb0e4ab3c56dae494bc0ca58c88526305d2 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Sun, 13 Sep 2026 07:20:58 +0300
Subject: [PATCH] gh-90304: Fix IDLE startup failure with a broken font
 (GH-152419)

On some systems a font reports a zero width for the '0' character (seen with a broken font on openSUSE). EditorWindow.set_width() divides the Text widget's pixel width by that value, so the ZeroDivisionError prevented IDLE from starting at all.

set_width() now falls back to the configured width when the measured width is zero. self.width is consumed by the === RESTART === separator in the shell and by the squeezer's line wrapping.
(cherry picked from commit 8a2db991dabe7ee5afd8385ef569f6e1f0120d6f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
---
 Lib/idlelib/editor.py                               |  4 +++-
 Lib/idlelib/idle_test/test_editor.py                | 13 +++++++++++++
 .../2026-06-27-17-27-21.gh-issue-90304.464d22.rst   |  4 ++++
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/IDLE/2026-06-27-17-27-21.gh-issue-90304.464d22.rst

diff --git a/Lib/idlelib/editor.py b/Lib/idlelib/editor.py
index 2b4b95e053ba10b..5e9f6aa86e81925 100644
--- a/Lib/idlelib/editor.py
+++ b/Lib/idlelib/editor.py
@@ -325,7 +325,9 @@ 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()
diff --git a/Lib/idlelib/idle_test/test_editor.py b/Lib/idlelib/idle_test/test_editor.py
index 873637f67defa37..e32981091b72a6e 100644
--- a/Lib/idlelib/idle_test/test_editor.py
+++ b/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):
diff --git a/Misc/NEWS.d/next/IDLE/2026-06-27-17-27-21.gh-issue-90304.464d22.rst b/Misc/NEWS.d/next/IDLE/2026-06-27-17-27-21.gh-issue-90304.464d22.rst
new file mode 100644
index 000000000000000..c92c1f75b9d7a7d
--- /dev/null
+++ b/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.