diff --git a/PowerEditor/scintilla.original.forUpdating/scintilla542.zip b/PowerEditor/scintilla.original.forUpdating/scintilla543.zip
similarity index 79%
rename from PowerEditor/scintilla.original.forUpdating/scintilla542.zip
rename to PowerEditor/scintilla.original.forUpdating/scintilla543.zip
index 513c8a305..808a09762 100644
Binary files a/PowerEditor/scintilla.original.forUpdating/scintilla542.zip and b/PowerEditor/scintilla.original.forUpdating/scintilla543.zip differ
diff --git a/scintilla/cocoa/Scintilla/Info.plist b/scintilla/cocoa/Scintilla/Info.plist
index c7533ddc3..a1d0470fc 100644
--- a/scintilla/cocoa/Scintilla/Info.plist
+++ b/scintilla/cocoa/Scintilla/Info.plist
@@ -15,7 +15,7 @@
 	<key>CFBundlePackageType</key>
 	<string>$(PRODUCT_BUNDLE_PACKAGE_TYPE)</string>
 	<key>CFBundleShortVersionString</key>
-	<string>5.4.2</string>
+	<string>5.4.3</string>
 	<key>CFBundleVersion</key>
 	<string>$(CURRENT_PROJECT_VERSION)</string>
 	<key>NSHumanReadableCopyright</key>
diff --git a/scintilla/cocoa/Scintilla/Scintilla.xcodeproj/project.pbxproj b/scintilla/cocoa/Scintilla/Scintilla.xcodeproj/project.pbxproj
index c30155539..7b4ea5f5f 100644
--- a/scintilla/cocoa/Scintilla/Scintilla.xcodeproj/project.pbxproj
+++ b/scintilla/cocoa/Scintilla/Scintilla.xcodeproj/project.pbxproj
@@ -582,7 +582,7 @@
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
 				COPY_PHASE_STRIP = NO;
-				CURRENT_PROJECT_VERSION = 5.4.2;
+				CURRENT_PROJECT_VERSION = 5.4.3;
 				DEAD_CODE_STRIPPING = YES;
 				DEBUG_INFORMATION_FORMAT = dwarf;
 				ENABLE_STRICT_OBJC_MSGSEND = YES;
@@ -647,7 +647,7 @@
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
 				COPY_PHASE_STRIP = NO;
-				CURRENT_PROJECT_VERSION = 5.4.2;
+				CURRENT_PROJECT_VERSION = 5.4.3;
 				DEAD_CODE_STRIPPING = YES;
 				DEBUG_INFORMATION_FORMAT = "dwarf-with-dsym";
 				ENABLE_NS_ASSERTIONS = NO;
@@ -680,7 +680,7 @@
 				CODE_SIGN_IDENTITY = "-";
 				CODE_SIGN_STYLE = Manual;
 				COMBINE_HIDPI_IMAGES = YES;
-				CURRENT_PROJECT_VERSION = 5.4.2;
+				CURRENT_PROJECT_VERSION = 5.4.3;
 				DEAD_CODE_STRIPPING = YES;
 				DEFINES_MODULE = YES;
 				DEVELOPMENT_TEAM = "";
@@ -715,7 +715,7 @@
 				CODE_SIGN_IDENTITY = "-";
 				CODE_SIGN_STYLE = Manual;
 				COMBINE_HIDPI_IMAGES = YES;
-				CURRENT_PROJECT_VERSION = 5.4.2;
+				CURRENT_PROJECT_VERSION = 5.4.3;
 				DEAD_CODE_STRIPPING = YES;
 				DEFINES_MODULE = YES;
 				DEVELOPMENT_TEAM = "";
diff --git a/scintilla/doc/ScintillaDoc.html b/scintilla/doc/ScintillaDoc.html
index e21536370..87bab3cd7 100644
--- a/scintilla/doc/ScintillaDoc.html
+++ b/scintilla/doc/ScintillaDoc.html
@@ -6420,6 +6420,18 @@ struct Sci_TextToFindFull {
 	  This also avoids a header rectangle above the list.</td>
         </tr>
 
+        <tr>
+          <td align="left"><code>SC_AUTOCOMPLETE_SELECT_FIRST_ITEM</code></td>
+
+          <td align="center">2</td>
+
+          <td>Always select the first item from the autocompletion list regardless of the value
+            entered in the editor. Useful when the autocompletion logic of the application
+            sorts autocompletion entries so that the best match is always at the top of the
+            list. Without this option, Scintilla selects the item from the autocompletion
+            list matching the value entered in the editor.</td>
+        </tr>
+
       </tbody>
     </table>
 
diff --git a/scintilla/doc/ScintillaDownload.html b/scintilla/doc/ScintillaDownload.html
index 2ebe36f1a..e3db879fe 100644
--- a/scintilla/doc/ScintillaDownload.html
+++ b/scintilla/doc/ScintillaDownload.html
@@ -26,9 +26,9 @@
     <table bgcolor="#CCCCCC" width="100%" cellspacing="0" cellpadding="8" border="0">
       <tr>
         <td>
-          <font size="4"> <a href="https://www.scintilla.org/scintilla542.zip">
+          <font size="4"> <a href="https://www.scintilla.org/scintilla543.zip">
 	Windows</a>&nbsp;&nbsp;
-	<a href="https://www.scintilla.org/scintilla542.tgz">
+	<a href="https://www.scintilla.org/scintilla543.tgz">
           GTK/Linux</a>&nbsp;&nbsp;
 	</font>
         </td>
@@ -42,7 +42,7 @@
       containing very few restrictions.
     </p>
     <h3>
-       Release 5.4.2
+       Release 5.4.3
     </h3>
     <h4>
        Source Code
@@ -50,8 +50,8 @@
        The source code package contains all of the source code for Scintilla but no binary
 	executable code and is available in
        <ul>
-       <li><a href="https://www.scintilla.org/scintilla542.zip">zip format</a> (1.8M) commonly used on Windows</li>
-       <li><a href="https://www.scintilla.org/scintilla542.tgz">tgz format</a> (1.7M) commonly used on Linux and compatible operating systems</li>
+       <li><a href="https://www.scintilla.org/scintilla543.zip">zip format</a> (1.8M) commonly used on Windows</li>
+       <li><a href="https://www.scintilla.org/scintilla543.tgz">tgz format</a> (1.7M) commonly used on Linux and compatible operating systems</li>
        </ul>
        Instructions for building on both Windows and Linux are included in the readme file.
     <h4>
diff --git a/scintilla/doc/ScintillaHistory.html b/scintilla/doc/ScintillaHistory.html
index 827ec6551..023c9a5a1 100644
--- a/scintilla/doc/ScintillaHistory.html
+++ b/scintilla/doc/ScintillaHistory.html
@@ -583,6 +583,23 @@
     </tr>
     </table>
     <h2>Releases</h2>
+    <h3>
+       <a href="https://www.scintilla.org/scintilla543.zip">Release 5.4.3</a>
+    </h3>
+    <ul>
+	<li>
+	Released 9 March 2024.
+	</li>
+	<li>
+	Fix redo failure introduced with 5.4.2.
+	<a href="https://sourceforge.net/p/scintilla/bugs/2432/">Bug #2432</a>.
+	</li>
+	<li>
+	Add SC_AUTOCOMPLETE_SELECT_FIRST_ITEM option to
+	always selects the first item in the autocompletion list.
+	<a href="https://sourceforge.net/p/scintilla/bugs/2403/">Bug #2403</a>.
+	</li>
+    </ul>
     <h3>
        <a href="https://www.scintilla.org/scintilla542.zip">Release 5.4.2</a>
     </h3>
@@ -12833,7 +12850,7 @@
         Pascal lexer handles comments more correctly.
       </li>
       <li>
-        C/C++/Java/JavaScipt lexer has a state for line doc comments.
+        C/C++/Java/JavaScript lexer has a state for line doc comments.
       </li>
       <li>
         Error output lexer understands Sun CC messages.
diff --git a/scintilla/doc/index.html b/scintilla/doc/index.html
index 565f88578..04b7cae38 100644
--- a/scintilla/doc/index.html
+++ b/scintilla/doc/index.html
@@ -9,7 +9,7 @@
     <meta name="keywords" content="Scintilla, SciTE, Editing Component, Text Editor" />
     <meta name="Description"
     content="www.scintilla.org is the home of the Scintilla editing component and SciTE text editor application." />
-    <meta name="Date.Modified" content="20240305" />
+    <meta name="Date.Modified" content="20240309" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <style type="text/css">
         .logo {
@@ -60,8 +60,8 @@
           GTK, and macOS</font>
         </td>
         <td width="40%" align="right">
-          <font color="#FFCC99" size="3"> Release version 5.4.2<br />
-           Site last modified March 5 2024</font>
+          <font color="#FFCC99" size="3"> Release version 5.4.3<br />
+           Site last modified March 9 2024</font>
         </td>
         <td width="20%">
           &nbsp;
@@ -76,11 +76,11 @@
       </tr>
     </table>
     <ul id="versionlist">
+      <li>Version 5.4.3 fixes a redo bug.</li>
       <li>Version 5.4.2 can save and restore undo history.</li>
       <li>Version 5.4.1 adds IDocumentEditable interface to allow efficient interaction with document objects.</li>
       <li>Version 5.4.0 fixes crashes on macOS 12 and older when built with Xcode 15.0.</li>
       <li>Version 5.3.8 fixes excesssive memory use when deleting contiguous ranges backwards and is compatible with new macOS 14.</li>
-      <li>Version 5.3.7 fixes platform-specific issues on GTK, Qt, and Win32.</li>
     </ul>
     <ul id="menu">
       <li id="remote1"><a href="https://www.scintilla.org/SciTEImage.html">Screenshot</a></li>
diff --git a/scintilla/include/Scintilla.h b/scintilla/include/Scintilla.h
index 1dd69713e..4d0ed13dc 100644
--- a/scintilla/include/Scintilla.h
+++ b/scintilla/include/Scintilla.h
@@ -446,6 +446,7 @@ typedef sptr_t (*SciFnDirectStatus)(sptr_t ptr, unsigned int iMessage, uptr_t wP
 #define SCI_AUTOCGETAUTOHIDE 2119
 #define SC_AUTOCOMPLETE_NORMAL 0
 #define SC_AUTOCOMPLETE_FIXED_SIZE 1
+#define SC_AUTOCOMPLETE_SELECT_FIRST_ITEM 2
 #define SCI_AUTOCSETOPTIONS 2638
 #define SCI_AUTOCGETOPTIONS 2639
 #define SCI_AUTOCSETDROPRESTOFWORD 2270
diff --git a/scintilla/include/Scintilla.iface b/scintilla/include/Scintilla.iface
index 442f1772c..87aa8c204 100644
--- a/scintilla/include/Scintilla.iface
+++ b/scintilla/include/Scintilla.iface
@@ -1100,6 +1100,8 @@ enu AutoCompleteOption=SC_AUTOCOMPLETE_
 val SC_AUTOCOMPLETE_NORMAL=0
 # Win32 specific:
 val SC_AUTOCOMPLETE_FIXED_SIZE=1
+# Always select the first item in the autocompletion list:
+val SC_AUTOCOMPLETE_SELECT_FIRST_ITEM=2
 
 # Set autocompletion options.
 set void AutoCSetOptions=2638(AutoCompleteOption options,)
diff --git a/scintilla/include/ScintillaTypes.h b/scintilla/include/ScintillaTypes.h
index ad792dd56..35d24a795 100644
--- a/scintilla/include/ScintillaTypes.h
+++ b/scintilla/include/ScintillaTypes.h
@@ -250,6 +250,7 @@ enum class IndicFlag {
 enum class AutoCompleteOption {
 	Normal = 0,
 	FixedSize = 1,
+	SelectFirstItem = 2,
 };
 
 enum class IndentView {
diff --git a/scintilla/qt/ScintillaEdit/ScintillaEdit.pro b/scintilla/qt/ScintillaEdit/ScintillaEdit.pro
index 105068363..b8d032085 100644
--- a/scintilla/qt/ScintillaEdit/ScintillaEdit.pro
+++ b/scintilla/qt/ScintillaEdit/ScintillaEdit.pro
@@ -13,7 +13,7 @@ TEMPLATE = lib
 CONFIG += lib_bundle
 CONFIG += c++1z
 
-VERSION = 5.4.2
+VERSION = 5.4.3
 
 SOURCES += \
     ScintillaEdit.cpp \
diff --git a/scintilla/qt/ScintillaEditBase/ScintillaEditBase.pro b/scintilla/qt/ScintillaEditBase/ScintillaEditBase.pro
index cabd4bf9c..23548df72 100644
--- a/scintilla/qt/ScintillaEditBase/ScintillaEditBase.pro
+++ b/scintilla/qt/ScintillaEditBase/ScintillaEditBase.pro
@@ -13,7 +13,7 @@ TEMPLATE = lib
 CONFIG += lib_bundle
 CONFIG += c++1z
 
-VERSION = 5.4.2
+VERSION = 5.4.3
 
 SOURCES += \
     PlatQt.cpp \
diff --git a/scintilla/src/CellBuffer.cxx b/scintilla/src/CellBuffer.cxx
index 4fcc2117d..8224ab286 100644
--- a/scintilla/src/CellBuffer.cxx
+++ b/scintilla/src/CellBuffer.cxx
@@ -47,7 +47,7 @@ struct CountWidths {
 		countOtherPlanes(countOtherPlanes_) {
 	}
 	CountWidths operator-() const noexcept {
-		return CountWidths(-countBasePlane , -countOtherPlanes);
+		return CountWidths(-countBasePlane, -countOtherPlanes);
 	}
 	Sci::Position WidthUTF32() const noexcept {
 		// All code points take one code unit in UTF-32.
@@ -832,7 +832,7 @@ void CellBuffer::BasicInsertString(Sci::Position position, const char *s, Sci::P
 	const Sci::Line lineStart = lineInsert;
 
 	// s may not NULL-terminated, ensure *ptr == '\n' or *next == '\n' is valid.
-	const char * const end = s + insertLength - 1;
+	const char *const end = s + insertLength - 1;
 	const char *ptr = s;
 	unsigned char ch = 0;
 
diff --git a/scintilla/src/CharacterCategoryMap.cxx b/scintilla/src/CharacterCategoryMap.cxx
index b8b20fee7..3ef5bb748 100644
--- a/scintilla/src/CharacterCategoryMap.cxx
+++ b/scintilla/src/CharacterCategoryMap.cxx
@@ -4050,12 +4050,17 @@ constexpr int maskCategory = 0x1F;
 // possibly for 0..0xff for most Western European text or 0..0xfff for most
 // alphabetic languages.
 
-CharacterCategory CategoriseCharacter(int character) {
+CharacterCategory CategoriseCharacter(int character) noexcept {
 	if (character < 0 || character > maxUnicode)
 		return ccCn;
 	const int baseValue = character * (maskCategory+1) + maskCategory;
-	const int *placeAfter = std::lower_bound(catRanges, std::end(catRanges), baseValue);
-	return static_cast<CharacterCategory>(*(placeAfter-1) & maskCategory);
+	try {
+		// lower_bound will never throw with these args but its not marked noexcept so add catch to pretend.
+		const int *placeAfter = std::lower_bound(catRanges, std::end(catRanges), baseValue);
+		return static_cast<CharacterCategory>(*(placeAfter - 1) & maskCategory);
+	} catch (...) {
+		return ccCn;
+	}
 }
 
 // Implementation of character sets recommended for identifiers in Unicode Standard Annex #31.
@@ -4158,7 +4163,7 @@ bool OmitXidContinue(int character) noexcept {
 
 // UAX #31 defines ID_Start as
 // [[:L:][:Nl:][:Other_ID_Start:]--[:Pattern_Syntax:]--[:Pattern_White_Space:]]
-bool IsIdStart(int character) {
+bool IsIdStart(int character) noexcept {
 	if (IsIdPattern(character)) {
 		return false;
 	}
@@ -4173,7 +4178,7 @@ bool IsIdStart(int character) {
 
 // UAX #31 defines ID_Continue as
 // [[:ID_Start:][:Mn:][:Mc:][:Nd:][:Pc:][:Other_ID_Continue:]--[:Pattern_Syntax:]--[:Pattern_White_Space:]]
-bool IsIdContinue(int character) {
+bool IsIdContinue(int character) noexcept {
 	if (IsIdPattern(character)) {
 		return false;
 	}
@@ -4187,7 +4192,7 @@ bool IsIdContinue(int character) {
 }
 
 // XID_Start is ID_Start modified for Normalization Form KC in UAX #31
-bool IsXidStart(int character) {
+bool IsXidStart(int character) noexcept {
 	if (OmitXidStart(character)) {
 		return false;
 	} else {
@@ -4196,7 +4201,7 @@ bool IsXidStart(int character) {
 }
 
 // XID_Continue is ID_Continue modified for Normalization Form KC in UAX #31
-bool IsXidContinue(int character) {
+bool IsXidContinue(int character) noexcept {
 	if (OmitXidContinue(character)) {
 		return false;
 	} else {
diff --git a/scintilla/src/CharacterCategoryMap.h b/scintilla/src/CharacterCategoryMap.h
index b3b03eb5a..cd75ec29c 100644
--- a/scintilla/src/CharacterCategoryMap.h
+++ b/scintilla/src/CharacterCategoryMap.h
@@ -22,20 +22,20 @@ enum CharacterCategory {
 	ccCc, ccCf, ccCs, ccCo, ccCn
 };
 
-CharacterCategory CategoriseCharacter(int character);
+CharacterCategory CategoriseCharacter(int character) noexcept;
 
 // Common definitions of allowable characters in identifiers from UAX #31.
-bool IsIdStart(int character);
-bool IsIdContinue(int character);
-bool IsXidStart(int character);
-bool IsXidContinue(int character);
+bool IsIdStart(int character) noexcept;
+bool IsIdContinue(int character) noexcept;
+bool IsXidStart(int character) noexcept;
+bool IsXidContinue(int character) noexcept;
 
 class CharacterCategoryMap {
 private:
 	std::vector<unsigned char> dense;
 public:
 	CharacterCategoryMap();
-	CharacterCategory CategoryFor(int character) const {
+	CharacterCategory CategoryFor(int character) const noexcept {
 		if (static_cast<size_t>(character) < dense.size()) {
 			return static_cast<CharacterCategory>(dense[character]);
 		} else {
diff --git a/scintilla/src/Document.cxx b/scintilla/src/Document.cxx
index f53bfe421..a4490a27a 100644
--- a/scintilla/src/Document.cxx
+++ b/scintilla/src/Document.cxx
@@ -1405,7 +1405,7 @@ IDocumentEditable *Document::AsDocumentEditable() noexcept {
 	return static_cast<IDocumentEditable *>(this);
 }
 
-void * SCI_METHOD Document::ConvertToDocument() {
+void *SCI_METHOD Document::ConvertToDocument() {
 	return AsDocumentEditable();
 }
 
@@ -1453,7 +1453,7 @@ Sci::Position Document::Undo() {
 					}
 				} else if (action.at == ActionType::insert) {
 					modFlags |= ModificationFlags::DeleteText;
-					coalescedRemove = Range(); 
+					coalescedRemove = Range();
 				}
 				if (steps > 1)
 					modFlags |= ModificationFlags::MultiStepUndoRedo;
@@ -2380,15 +2380,15 @@ void Document::AllocateLines(Sci::Line lines) {
 }
 
 void Document::SetDefaultCharClasses(bool includeWordClass) {
-    charClass.SetDefaultCharClasses(includeWordClass);
+	charClass.SetDefaultCharClasses(includeWordClass);
 }
 
 void Document::SetCharClasses(const unsigned char *chars, CharacterClass newCharClass) {
-    charClass.SetCharClasses(chars, newCharClass);
+	charClass.SetCharClasses(chars, newCharClass);
 }
 
 int Document::GetCharsOfClass(CharacterClass characterClass, unsigned char *buffer) const {
-    return charClass.GetCharsOfClass(characterClass, buffer);
+	return charClass.GetCharsOfClass(characterClass, buffer);
 }
 
 void Document::SetCharacterCategoryOptimization(int countCharacters) {
diff --git a/scintilla/src/Document.h b/scintilla/src/Document.h
index a42bfc301..da3736cc6 100644
--- a/scintilla/src/Document.h
+++ b/scintilla/src/Document.h
@@ -385,7 +385,7 @@ public:
 	void ChangeInsertion(const char *s, Sci::Position length);
 	int SCI_METHOD AddData(const char *data, Sci_Position length) override;
 	IDocumentEditable *AsDocumentEditable() noexcept;
-	void * SCI_METHOD ConvertToDocument() override;
+	void *SCI_METHOD ConvertToDocument() override;
 	Sci::Position Undo();
 	Sci::Position Redo();
 	bool CanUndo() const noexcept { return cb.CanUndo(); }
@@ -427,7 +427,7 @@ public:
 	[[nodiscard]] unsigned int EditionDeletesAt(Sci::Position pos) const noexcept { return cb.EditionDeletesAt(pos); }
 	[[nodiscard]] Sci::Position EditionNextDelete(Sci::Position pos) const noexcept { return cb.EditionNextDelete(pos); }
 
-	const char * SCI_METHOD BufferPointer() override { return cb.BufferPointer(); }
+	const char *SCI_METHOD BufferPointer() override { return cb.BufferPointer(); }
 	const char *RangePointer(Sci::Position position, Sci::Position rangeLength) noexcept { return cb.RangePointer(position, rangeLength); }
 	Sci::Position GapPosition() const noexcept { return cb.GapPosition(); }
 
diff --git a/scintilla/src/Editor.cxx b/scintilla/src/Editor.cxx
index 06d46ef6a..0484517cc 100644
--- a/scintilla/src/Editor.cxx
+++ b/scintilla/src/Editor.cxx
@@ -1915,7 +1915,7 @@ Sci::Position Editor::FormatRange(Scintilla::Message iMessage, Scintilla::uptr_t
 	void *ptr = PtrFromSPtr(lParam);
 	if (iMessage == Message::FormatRange) {
 		RangeToFormat *pfr = static_cast<RangeToFormat *>(ptr);
-		const CharacterRangeFull chrg{ pfr->chrg.cpMin,pfr->chrg.cpMax };
+		const CharacterRangeFull chrg{ pfr->chrg.cpMin, pfr->chrg.cpMax };
 		AutoSurface surface(pfr->hdc, this, Technology::Default);
 		AutoSurface surfaceMeasure(pfr->hdcTarget, this, Technology::Default);
 		if (!surface || !surfaceMeasure) {
diff --git a/scintilla/src/ScintillaBase.cxx b/scintilla/src/ScintillaBase.cxx
index 4e7e99581..288de6ef0 100644
--- a/scintilla/src/ScintillaBase.cxx
+++ b/scintilla/src/ScintillaBase.cxx
@@ -353,6 +353,8 @@ void ScintillaBase::AutoCompleteMove(int delta) {
 }
 
 void ScintillaBase::AutoCompleteMoveToCurrentWord() {
+	if (FlagSet(ac.options, AutoCompleteOption::SelectFirstItem))
+		return;
 	std::string wordCurrent = RangeText(ac.posStart - ac.startLen, sel.MainCaret());
 	ac.Select(wordCurrent.c_str());
 }
diff --git a/scintilla/src/UndoHistory.cxx b/scintilla/src/UndoHistory.cxx
index c94d0690c..daae97ad5 100644
--- a/scintilla/src/UndoHistory.cxx
+++ b/scintilla/src/UndoHistory.cxx
@@ -44,7 +44,6 @@ void VectorTruncate(std::vector<T> &v, size_t length) noexcept {
 
 constexpr size_t byteMask = UINT8_MAX;
 constexpr size_t byteBits = 8;
-constexpr size_t maxElementSize = 8;
 
 size_t ReadValue(const uint8_t *bytes, size_t length) noexcept {
 	size_t value = 0;
@@ -75,13 +74,13 @@ intptr_t ScaledVector::SignedValueAt(size_t index) const noexcept {
 
 constexpr SizeMax ElementForValue(size_t value) noexcept {
 	size_t maxN = byteMask;
-	for (size_t i = 1; i < maxElementSize; i++) {
-		if (value <= maxN) {
-			return { i, maxN };
-		}
+	size_t i = 1;
+	while (value > byteMask) {
+		i++;
+		value >>= byteBits;
 		maxN = (maxN << byteBits) + byteMask;
 	}
-	return { 1, byteMask };
+	return { i, maxN };
 }
 
 void ScaledVector::SetValueAt(size_t index, size_t value) {
@@ -102,7 +101,7 @@ void ScaledVector::SetValueAt(size_t index, size_t value) {
 }
 
 void ScaledVector::ClearValueAt(size_t index) noexcept {
-	// 0 fits in any size element so no expansion needed so no exceptions 
+	// 0 fits in any size element so no expansion needed so no exceptions
 	WriteValue(bytes.data() + index * element.size, element.size, 0);
 }
 
@@ -316,6 +315,8 @@ const char *UndoHistory::AppendAction(ActionType at, Sci::Position position, con
 	const char *dataNew = lengthData ? scraps->Push(data, lengthData) : nullptr;
 	if (currentAction >= actions.SSize()) {
 		actions.PushBack();
+	} else {
+		actions.Truncate(currentAction+1);
 	}
 	actions.Create(currentAction, at, position, lengthData, mayCoalesce);
 	currentAction++;
diff --git a/scintilla/test/simpleTests.py b/scintilla/test/simpleTests.py
index 0586f62f0..7d6de637f 100644
--- a/scintilla/test/simpleTests.py
+++ b/scintilla/test/simpleTests.py
@@ -3129,6 +3129,28 @@ class TestAutoComplete(unittest.TestCase):
 
 		self.assertEqual(self.ed.AutoCActive(), 0)
 
+	def testAutoSelectFirstItem(self):
+		self.assertEqual(self.ed.AutoCActive(), 0)
+
+		self.ed.AutoCSetOrder(self.ed.SC_ORDER_CUSTOM)
+
+		# without SC_AUTOCOMPLETE_SELECT_FIRST_ITEM option
+		self.ed.SetSel(3, 3)
+		self.ed.AutoCShow(3, b"aaa1 bbb1 xxx1")
+		# automatically selects the item with the entered prefix xxx
+		self.ed.AutoCComplete()
+		self.assertEqual(self.ed.Contents(), b"xxx1\n")
+
+		# with SC_AUTOCOMPLETE_SELECT_FIRST_ITEM option
+		self.ed.AutoCSetOptions(2, 0)
+		self.ed.SetSel(3, 3)
+		self.ed.AutoCShow(3, b"aaa1 bbb1 xxx1")
+		# selects the first item regardless of the entered prefix and replaces the entered xxx
+		self.ed.AutoCComplete()
+		self.assertEqual(self.ed.Contents(), b"aaa11\n")
+
+		self.assertEqual(self.ed.AutoCActive(), 0)
+
 	def testAutoCustomSort(self):
 		# Checks bug #2294 where SC_ORDER_CUSTOM with an empty list asserts
 		# https://sourceforge.net/p/scintilla/bugs/2294/
diff --git a/scintilla/test/unit/testCellBuffer.cxx b/scintilla/test/unit/testCellBuffer.cxx
index a6bd5dba1..acd21d32e 100644
--- a/scintilla/test/unit/testCellBuffer.cxx
+++ b/scintilla/test/unit/testCellBuffer.cxx
@@ -397,6 +397,32 @@ TEST_CASE("UndoHistory") {
 		REQUIRE(!uh.IsSavePoint());
 	}
 
+	SECTION("EnsureTruncationAfterUndo") {
+
+		REQUIRE(uh.Actions() == 0);
+		bool startSequence = false;
+		uh.AppendAction(ActionType::insert, 0, "ab", 2, startSequence, true);
+		REQUIRE(uh.Actions() == 1);
+		uh.AppendAction(ActionType::insert, 2, "cd", 2, startSequence, true);
+		REQUIRE(uh.Actions() == 2);
+		REQUIRE(uh.CanUndo());
+		REQUIRE(!uh.CanRedo());
+
+		// Undoing
+		const int steps = uh.StartUndo();
+		REQUIRE(steps == 2);
+		uh.GetUndoStep();
+		uh.CompletedUndoStep();
+		REQUIRE(uh.Actions() == 2);	// Not truncated until forward action
+		uh.GetUndoStep();
+		uh.CompletedUndoStep();
+		REQUIRE(uh.Actions() == 2);
+
+		// Perform action which should truncate history
+		uh.AppendAction(ActionType::insert, 0, "12", 2, startSequence, true);
+		REQUIRE(uh.Actions() == 1);
+	}
+
 	SECTION("Coalesce") {
 
 		bool startSequence = false;
diff --git a/scintilla/version.txt b/scintilla/version.txt
index 32203d46c..05c186596 100644
--- a/scintilla/version.txt
+++ b/scintilla/version.txt
@@ -1 +1 @@
-542
+543
diff --git a/scintilla/win32/ScintRes.rc b/scintilla/win32/ScintRes.rc
index 08a73d9aa..aa29e8f12 100644
--- a/scintilla/win32/ScintRes.rc
+++ b/scintilla/win32/ScintRes.rc
@@ -4,8 +4,8 @@
 
 #include <windows.h>
 
-#define VERSION_SCINTILLA "5.4.2"
-#define VERSION_WORDS 5, 4, 2, 0
+#define VERSION_SCINTILLA "5.4.3"
+#define VERSION_WORDS 5, 4, 3, 0
 
 VS_VERSION_INFO VERSIONINFO
 FILEVERSION	VERSION_WORDS