From 724429a7c661eeafe3094a049f747267d0496a69 Mon Sep 17 00:00:00 2001
From: Neil <nyamatongwe@gmail.com>
Date: Mon, 8 Dec 2025 14:38:07 +1100
Subject: Bug [#2488]. Fix SCI_SETSELECTIONNSTART and SCI_SETSELECTIONNEND.

---
 doc/ScintillaHistory.html   | 12 ++++++
 src/Editor.cxx              |  4 +-
 src/Selection.cxx           | 28 ++++++++++++++
 src/Selection.h             |  2 +
 test/simpleTests.py         | 22 +++++++++++
 test/unit/testSelection.cxx | 90 +++++++++++++++++++++++++++++++++++++++++++++
 6 files changed, 156 insertions(+), 2 deletions(-)

diff --git a/doc/ScintillaHistory.html b/doc/ScintillaHistory.html
index 0cced840e..48529c8d6 100644
--- a/doc/ScintillaHistory.html
+++ b/doc/ScintillaHistory.html
@@ -594,6 +594,18 @@
     </tr>
     </table>
     <h2 id="Releases">Releases</h2>
+    <h3>
+       <a href="https://www.scintilla.org/scintilla559.zip">Release 5.5.9</a>
+    </h3>
+    <ul>
+	<li>
+	Released 10 November 2025.
+	</li>
+	<li>
+	Fix SCI_SETSELECTIONNSTART and SCI_SETSELECTIONNEND to behave more sensibly.
+	<a href="https://sourceforge.net/p/scintilla/bugs/2488/">Bug #2488</a>.
+	</li>
+    </ul>
     <h3>
        <a href="https://www.scintilla.org/scintilla558.zip">Release 5.5.8</a>
     </h3>
diff --git a/src/Editor.cxx b/src/Editor.cxx
index e7a309e67..1ad99c3ca 100644
--- a/src/Editor.cxx
+++ b/src/Editor.cxx
@@ -6178,11 +6178,11 @@ void Editor::SetSelectionNMessage(Message iMessage, uptr_t wParam, sptr_t lParam
 		break;
 
 	case Message::SetSelectionNStart:
-		sel.Range(wParam).anchor.SetPosition(lParam);
+		sel.Range(wParam).StartSet(SelectionPosition(lParam));
 		break;
 
 	case Message::SetSelectionNEnd:
-		sel.Range(wParam).caret.SetPosition(lParam);
+		sel.Range(wParam).EndSet(SelectionPosition(lParam));
 		break;
 
 	default:
diff --git a/src/Selection.cxx b/src/Selection.cxx
index 967033bff..e8d0148f5 100644
--- a/src/Selection.cxx
+++ b/src/Selection.cxx
@@ -182,6 +182,34 @@ SelectionSegment SelectionRange::Intersect(SelectionSegment check) const noexcep
 	};
 }
 
+void SelectionRange::StartSet(SelectionPosition sp) noexcept {
+	if (anchor <= caret) {
+		anchor = sp;
+		if (caret < anchor) {
+			caret = anchor;
+		}
+	} else {
+		caret = sp;
+		if (anchor < caret) {
+			anchor = caret;
+		}
+	}
+}
+
+void SelectionRange::EndSet(SelectionPosition sp) noexcept {
+	if (caret >= anchor) {
+		caret = sp;
+		if (anchor > caret) {
+			anchor = caret;
+		}
+	} else {
+		anchor = sp;
+		if (caret > anchor) {
+			caret = anchor;
+		}
+	}
+}
+
 void SelectionRange::Swap() noexcept {
 	std::swap(caret, anchor);
 }
diff --git a/src/Selection.h b/src/Selection.h
index 1a57d2e78..0b3bb9159 100644
--- a/src/Selection.h
+++ b/src/Selection.h
@@ -156,6 +156,8 @@ struct SelectionRange {
 	SelectionPosition End() const noexcept {
 		return (anchor < caret) ? caret : anchor;
 	}
+	void StartSet(SelectionPosition sp) noexcept;
+	void EndSet(SelectionPosition sp) noexcept;
 	void Swap() noexcept;
 	bool Trim(SelectionRange range) noexcept;
 	void Truncate(Sci::Position length) noexcept;
diff --git a/test/simpleTests.py b/test/simpleTests.py
index 020b72e99..4e6a51d59 100644
--- a/test/simpleTests.py
+++ b/test/simpleTests.py
@@ -1967,6 +1967,28 @@ class TestMultiSelection(unittest.TestCase):
 		self.assertEqual(self.ed.GetSelectionNStart(0), 2)
 		self.assertEqual(self.ed.GetSelectionNEnd(0), 3)
 
+		self.ed.SetSelectionNStart(0, 1)
+		self.assertEqual(self.ed.GetSelectionNAnchor(0), 1)
+		self.assertEqual(self.ed.GetSelectionNCaret(0), 3)
+		self.assertEqual(self.ed.GetSelectionNStart(0), 1)
+		self.assertEqual(self.ed.GetSelectionNEnd(0), 3)
+
+		self.ed.SetSelectionNAnchor(0, 2)
+		self.ed.SetSelectionNCaret(0, 2)
+		self.ed.SetSelectionNStart(0, 9)
+		self.assertEqual(self.ed.GetSelectionNAnchor(0), 9)
+		self.assertEqual(self.ed.GetSelectionNCaret(0), 9)
+		self.assertEqual(self.ed.GetSelectionNStart(0), 9)
+		self.assertEqual(self.ed.GetSelectionNEnd(0), 9)
+
+		self.ed.SetSelectionNAnchor(0, 2)
+		self.ed.SetSelectionNCaret(0, 3)
+		self.ed.SetSelectionNStart(0, 9)
+		self.assertEqual(self.ed.GetSelectionNAnchor(0), 9)
+		self.assertEqual(self.ed.GetSelectionNCaret(0), 9)
+		self.assertEqual(self.ed.GetSelectionNStart(0), 9)
+		self.assertEqual(self.ed.GetSelectionNEnd(0), 9)
+
 	def test2Selections(self):
 		self.ed.SetSelection(1, 2)
 		self.ed.AddSelection(4, 5)
diff --git a/test/unit/testSelection.cxx b/test/unit/testSelection.cxx
index ab0065624..76eebca21 100644
--- a/test/unit/testSelection.cxx
+++ b/test/unit/testSelection.cxx
@@ -198,6 +198,96 @@ TEST_CASE("SelectionRange") {
 		REQUIRE(thin == single);
 	}
 
+	SECTION("StartEndSet") {
+		{
+			SelectionRange range;
+
+			range.StartSet(SelectionPosition(2));
+			range.EndSet(SelectionPosition(3));
+			REQUIRE(range.Start() == SelectionPosition(2));
+			REQUIRE(range.End() == SelectionPosition(3));
+			REQUIRE(range == SelectionRange(3, 2));
+
+			range.StartSet(SelectionPosition(1));
+			REQUIRE(range.Start() == SelectionPosition(1));
+			REQUIRE(range.End() == SelectionPosition(3));
+			REQUIRE(range == SelectionRange(3, 1));
+		}
+
+		{
+			// Outside after
+			SelectionRange range(2, 1);
+			range.StartSet(SelectionPosition(3));
+			REQUIRE(range.Start() == SelectionPosition(3));
+			REQUIRE(range.End() == SelectionPosition(3));
+			REQUIRE(range == SelectionRange(3, 3));
+		}
+
+		{
+			// Outside after
+			SelectionRange range(2, 1);
+			range.EndSet(SelectionPosition(3));
+			REQUIRE(range.Start() == SelectionPosition(1));
+			REQUIRE(range.End() == SelectionPosition(3));
+			REQUIRE(range == SelectionRange(3, 1));
+		}
+
+		{
+			// Outside before
+			SelectionRange range(2, 1);
+			range.StartSet(SelectionPosition(0));
+			REQUIRE(range.Start() == SelectionPosition(0));
+			REQUIRE(range.End() == SelectionPosition(2));
+			REQUIRE(range == SelectionRange(2, 0));
+		}
+
+		{
+			// Outside before
+			SelectionRange range(2, 1);
+			range.EndSet(SelectionPosition(0));
+			REQUIRE(range.Start() == SelectionPosition(0));
+			REQUIRE(range.End() == SelectionPosition(0));
+			REQUIRE(range == SelectionRange(0, 0));
+		}
+
+		{
+			// Inside
+			SelectionRange range(3, 1);
+			range.EndSet(SelectionPosition(2));
+			REQUIRE(range.Start() == SelectionPosition(1));
+			REQUIRE(range.End() == SelectionPosition(2));
+			REQUIRE(range == SelectionRange(2, 1));
+		}
+
+		{
+			// Inside
+			SelectionRange range(3, 1);
+			range.StartSet(SelectionPosition(2));
+			REQUIRE(range.Start() == SelectionPosition(2));
+			REQUIRE(range.End() == SelectionPosition(3));
+			REQUIRE(range == SelectionRange(3, 2));
+		}
+
+		{
+			// Empty then outside
+			SelectionRange range(2);
+			range.StartSet(SelectionPosition(9));
+			REQUIRE(range.Start() == SelectionPosition(9));
+			REQUIRE(range.End() == SelectionPosition(9));
+			REQUIRE(range == SelectionRange(9, 9));
+		}
+
+		{
+			// Empty then outside
+			SelectionRange range(2);
+			range.StartSet(SelectionPosition(0));
+			REQUIRE(range.Start() == SelectionPosition(0));
+			REQUIRE(range.End() == SelectionPosition(2));
+			REQUIRE(range == SelectionRange(2, 0));
+		}
+
+	}
+
 }
 
 TEST_CASE("Selection") {
-- 
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