diff --git a/src/calibre/gui2/tweak_book/editor/text.py b/src/calibre/gui2/tweak_book/editor/text.py
index e74035293c..feb2de578b 100644
--- a/src/calibre/gui2/tweak_book/editor/text.py
+++ b/src/calibre/gui2/tweak_book/editor/text.py
@@ -179,8 +179,14 @@ class TextEdit(PlainTextEdit):
 
         def insert_text(text):
             c = self.textCursor()
-            c.insertText(text)
+            c.insertText(unicodedata.normalize('NFC', text))
             self.setTextCursor(c)
+            text = self.toPlainText()
+            if (ntext := unicodedata.normalize('NFC', text)) != text:
+                pos = c.position()
+                self.setPlainText(ntext)
+                c.setPosition(pos)
+                self.setTextCursor(c)
             self.ensureCursorVisible()
 
         def add_file(name, data, mt=None):
diff --git a/src/calibre/gui2/tweak_book/widgets.py b/src/calibre/gui2/tweak_book/widgets.py
index f4ba02680e..d090e7ec47 100644
--- a/src/calibre/gui2/tweak_book/widgets.py
+++ b/src/calibre/gui2/tweak_book/widgets.py
@@ -1274,6 +1274,9 @@ class PlainTextEdit(QPlainTextEdit):  # {{{
     def selected_text_from_cursor(self, cursor):
         return unicodedata.normalize('NFC', str(cursor.selectedText()).replace(PARAGRAPH_SEPARATOR, '\n').rstrip('\0'))
 
+    def paste(self):
+        super().paste()
+
     @property
     def selected_text(self):
         return self.selected_text_from_cursor(self.textCursor())