gh-75487: Do not save a truncated number when an int entry is blanked in IDLE Settings

The value is recorded on every keystroke, so blanking "80" with the
Backspace key left "8" to be saved.  Blanking an entry now forgets the
recorded value, so that the saved value is kept.

Author: serhiy-storchaka

Lib/idlelib/configdialog.py

@@ -2266,10 +2266,13 @@ def make_callback(var, config):
         def default_callback(*params):
             "Add config values to changes instance."
             value = var.get()
-            # A blanked int entry is an empty string; do not save it as an
-            # invalid config value (gh-83653).
             if value != '':
                 changes.add_option(*config, value)
+            else:
+                # A blanked int entry: do not save an invalid value, and
+                # forget the value recorded while editing (gh-75487).
+                config_type, section, item = config
+                changes[config_type].get(section, {}).pop(item, None)
         return default_callback
 
     def attach(self):

Lib/idlelib/idle_test/test_configdialog.py

@@ -1678,6 +1678,10 @@ def test_make_callback(self):
         sv.set('5')
         cb()
         self.assertEqual(changes['main']['section']['option'], '5')
+        # gh-75487: blanking the entry forgets the value recorded before.
+        sv.set('')
+        cb()
+        self.assertNotIn('option', changes['main']['section'])
         changes.clear()
 
     def test_attach_detach(self):

Misc/NEWS.d/next/IDLE/2026-09-17-17-00-00.gh-issue-75487.blankint.rst

@@ -0,0 +1,2 @@
+Fix saving a truncated number when an integer entry in the IDLE Settings
+dialog is blanked with the Backspace key.