From 0627663abd14c81cafde85ce9cc0502d9bc3a6c3 Mon Sep 17 00:00:00 2001
From: Neil <nyamatongwe@gmail.com>
Date: Wed, 7 Jan 2026 11:51:34 +1100
Subject: Bug [#2491]. Fix lexing after undo at end of document.

---
 doc/ScintillaHistory.html | 4 ++++
 src/Document.cxx          | 5 ++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/doc/ScintillaHistory.html b/doc/ScintillaHistory.html
index 25ca454cf..12b233b4f 100644
--- a/doc/ScintillaHistory.html
+++ b/doc/ScintillaHistory.html
@@ -607,6 +607,10 @@
 	<a href="https://sourceforge.net/p/scintilla/bugs/2488/">Bug #2488</a>.
 	</li>
 	<li>
+	Fix lexing after undo at end of document.
+	<a href="https://sourceforge.net/p/scintilla/bugs/2491/">Bug #2491</a>.
+	</li>
+	<li>
 	On Qt, add const to ScintillaDocument and ScintillaEdit methods.
 	<a href="https://sourceforge.net/p/scintilla/bugs/2494/">Bug #2494</a>.
 	</li>
diff --git a/src/Document.cxx b/src/Document.cxx
index 064b336e3..e4db66a19 100644
--- a/src/Document.cxx
+++ b/src/Document.cxx
@@ -1590,7 +1590,10 @@ Sci::Position Document::Undo() {
 				}
 				cb.PerformUndoStep();
 				if (action.at != ActionType::container) {
-					ModifiedAt(action.position);
+					if ((action.at == ActionType::insert) && (action.position >= LengthNoExcept()) && (action.position > 0))
+						ModifiedAt(action.position - 1);
+					else
+						ModifiedAt(action.position);
 					newPos = action.position;
 				}
 
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