Do not call addListener in constructors of CCombo and StyledText #2733

The method is not final, which may cause issues if a subclass overrides
it because it ends up being called upon instantiation, when the object
is not fully initialized

Since these 2 classes are not in the same package as other subclasses of
Widget, a new internal API was used.

Fixes #2733

Author: fedejeanne

bundles/org.eclipse.swt/Eclipse SWT Custom Widgets/common/org/eclipse/swt/custom/CCombo.java

@@ -173,7 +173,7 @@ public CCombo (Composite parent, int style) {
 	}
 
 	initAccessible();
-	addListener(SWT.ZoomChanged, this::handleDPIChange);
+	_addListenerFinal(SWT.ZoomChanged, this::handleDPIChange);
 }
 static int checkStyle (int style) {
 	int mask = SWT.BORDER | SWT.READ_ONLY | SWT.FLAT | SWT.LEFT_TO_RIGHT | SWT.RIGHT_TO_LEFT | SWT.LEAD | SWT.CENTER | SWT.TRAIL;

bundles/org.eclipse.swt/Eclipse SWT Custom Widgets/common/org/eclipse/swt/custom/StyledText.java

@@ -758,7 +758,7 @@ public StyledText(Composite parent, int style) {
 	initializeAccessible();
 	setData("DEFAULT_DROP_TARGET_EFFECT", new StyledTextDropTargetEffect(this));
 	if (IS_MAC) setData(STYLEDTEXT_KEY);
-	addListener(SWT.ZoomChanged, this::handleDPIChange);
+	_addListenerFinal(SWT.ZoomChanged, this::handleDPIChange);
 }
 /**
  * Adds an extended modify listener. An ExtendedModify event is sent by the

bundles/org.eclipse.swt/Eclipse SWT/cocoa/org/eclipse/swt/widgets/Widget.java

@@ -507,11 +507,40 @@ protected void addTypedListener (EventListener listener, int... eventTypes) {
 	}
 }
 
-void _addListener (int eventType, Listener listener) {
+/**
+ * Same as {@linkplain #_addListener(int, org.eclipse.swt.widgets.Listener)} but
+ * intentionally declared as {@code final} so subclasses may safely call it in
+ * their constructors.
+ *
+ * @see <a href=
+ *      "https://github.com/eclipse-platform/eclipse.platform.swt/issues/2733">#2733</a>
+ * @since 3.132
+ * @noreference This method is not intended to be referenced by clients.
+ */
+protected final void _addListenerFinal (int eventType, Listener listener) {
 	if (eventTable == null) eventTable = new EventTable ();
 	eventTable.hook (eventType, listener);
 }
 
+/**
+ * Same as {@linkplain #_removeListener(int, org.eclipse.swt.widgets.Listener)}
+ * but intentionally declared as {@code final} so subclasses may safely call it
+ * in their constructors.
+ *
+ * @see <a href=
+ *      "https://github.com/eclipse-platform/eclipse.platform.swt/issues/2733">#2733</a>
+ * @since 3.132
+ * @noreference This method is not intended to be referenced by clients.
+ */
+protected final void _removeListenerFinal (int eventType, Listener listener) {
+	if (eventTable == null) return;
+	eventTable.unhook (eventType, listener);
+}
+
+void _addListener (int eventType, Listener listener) {
+	_addListenerFinal(eventType, listener);
+}
+
 /**
  * Adds the listener to the collection of listeners who will
  * be notified when the widget is disposed. When the widget is

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Widget.java

@@ -297,11 +297,40 @@ public Widget (Widget parent, int style) {
 	notifyCreationTracker();
 }
 
-void _addListener (int eventType, Listener listener) {
+/**
+ * Same as {@linkplain #_addListener(int, org.eclipse.swt.widgets.Listener)} but
+ * intentionally declared as {@code final} so subclasses may safely call it in
+ * their constructors.
+ *
+ * @see <a href=
+ *      "https://github.com/eclipse-platform/eclipse.platform.swt/issues/2733">#2733</a>
+ * @since 3.132
+ * @noreference This method is not intended to be referenced by clients.
+ */
+protected final void _addListenerFinal (int eventType, Listener listener) {
 	if (eventTable == null) eventTable = new EventTable ();
 	eventTable.hook (eventType, listener);
 }
 
+/**
+ * Same as {@linkplain #_removeListener(int, org.eclipse.swt.widgets.Listener)}
+ * but intentionally declared as {@code final} so subclasses may safely call it
+ * in their constructors.
+ *
+ * @see <a href=
+ *      "https://github.com/eclipse-platform/eclipse.platform.swt/issues/2733">#2733</a>
+ * @since 3.132
+ * @noreference This method is not intended to be referenced by clients.
+ */
+protected final void _removeListenerFinal (int eventType, Listener listener) {
+	if (eventTable == null) return;
+	eventTable.unhook (eventType, listener);
+}
+
+void _addListener (int eventType, Listener listener) {
+	_addListenerFinal(eventType, listener);
+}
+
 /**
  * Adds the listener to the collection of {@link Listener listeners} who will
  * be notified when an event of the given type occurs. When the

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Widget.java

@@ -196,14 +196,42 @@ void registerDPIChangeListener() {
 	}
 }
 
-void _addListener (int eventType, Listener listener) {
+/**
+ * Same as {@linkplain #_addListener(int, org.eclipse.swt.widgets.Listener)} but
+ * intentionally declared as {@code final} so subclasses may safely call it in
+ * their constructors.
+ *
+ * @see <a href=
+ *      "https://github.com/eclipse-platform/eclipse.platform.swt/issues/2733">#2733</a>
+ * @since 3.132
+ * @noreference This method is not intended to be referenced by clients.
+ */
+protected final void _addListenerFinal (int eventType, Listener listener) {
 	if (eventTable == null) eventTable = new EventTable ();
 	eventTable.hook (eventType, listener);
 }
 
-void _removeListener (int eventType, Listener listener) {
+/**
+ * Same as {@linkplain #_removeListener(int, org.eclipse.swt.widgets.Listener)}
+ * but intentionally declared as {@code final} so subclasses may safely call it
+ * in their constructors.
+ *
+ * @see <a href=
+ *      "https://github.com/eclipse-platform/eclipse.platform.swt/issues/2733">#2733</a>
+ * @since 3.132
+ * @noreference This method is not intended to be referenced by clients.
+ */
+protected final void _removeListenerFinal(int eventType, Listener listener) {
 	if (eventTable == null) return;
-	eventTable.unhook (eventType, listener);
+	eventTable.unhook(eventType, listener);
+}
+
+void _addListener (int eventType, Listener listener) {
+	_addListenerFinal(eventType, listener);
+}
+
+void _removeListener (int eventType, Listener listener) {
+	_removeListenerFinal(eventType, listener);
 }
 
 /**