diff --git a/src/calibre/gui2/tweak_book/diff/highlight.py b/src/calibre/gui2/tweak_book/diff/highlight.py
index 52325b3e72..43abcaa5bb 100644
--- a/src/calibre/gui2/tweak_book/diff/highlight.py
+++ b/src/calibre/gui2/tweak_book/diff/highlight.py
@@ -31,6 +31,7 @@ class QtHighlighter(QTextDocument):
         ''' Copy specified lines from the syntax highlighted buffer into the
         destination cursor, preserving all formatting created by the syntax
         highlighter. '''
+        self.highlighter.join()
         num = hi - lo
         if num > 0:
             block = self.findBlockByNumber(lo)