Keep the text resize handles working after a scene change

A scene drops the event filters of an item that leaves it. The handles
installed theirs once, in the constructor, and only if the text was in
a scene: after the text was removed and added back (undoing a delete
does that), or when the handles were made before the text was in a
scene, they no longer reacted, and a drag moved the text instead.
TextResizeHandles now installs the filters again whenever it enters a
scene.

Tests: dragAfterLeavingTheScene, dragWhenCreatedOutsideAScene (both
fail before).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/QetGraphicsItemModeler/textresizehandles.cpp

@@ -26,11 +26,12 @@
 #include <QTextDocument>
 
 #include <cmath>
+#include <utility>
 
 /**
 	@brief TextResizeHandles::TextResizeHandles
 	@param text : the text to resize, becomes the parent of the handles.
-	It must be in a scene, the handles filter their own events.
+	The text may be in a scene or not yet.
 	@param handle_size : see QETUtils::graphicsHandlerSize()
 */
 TextResizeHandles::TextResizeHandles(QGraphicsTextItem *text, qreal handle_size) :
@@ -48,10 +49,9 @@ TextResizeHandles::TextResizeHandles(QGraphicsTextItem *text, qreal handle_size)
 							  "double-cliquer pour une largeur automatique"));
 		handle->setCursor(i == TextResize::TopLeft || i == TextResize::BottomRight
 						  ? Qt::SizeFDiagCursor : Qt::SizeBDiagCursor);
-		if (scene())
-			handle->installSceneEventFilter(this);
 		m_handles << handle;
 	}
+	installHandleFilters();
 
 	m_size_connection = connect(text->document()->documentLayout(),
 								&QAbstractTextDocumentLayout::documentSizeChanged,
@@ -90,6 +90,34 @@ void TextResizeHandles::updateHandlesPos()
 		m_handles.at(i)->setPos(TextResize::cornerOf(rect, TextResize::Corner(i)));
 }
 
+/**
+	@brief TextResizeHandles::itemChange
+	A scene drops the event filters of an item that leaves it, so the
+	handles stop reacting to the mouse when the text is removed from its
+	scene and added back (undoing a delete does that), or when they were
+	created before the text was in a scene. Install them again each time
+	these handles enter a scene.
+*/
+QVariant TextResizeHandles::itemChange(GraphicsItemChange change, const QVariant &value)
+{
+	if (change == QGraphicsItem::ItemSceneHasChanged)
+		installHandleFilters();
+	return QGraphicsObject::itemChange(change, value);
+}
+
+/**
+	@brief TextResizeHandles::installHandleFilters
+	Filter the mouse events of the handles, when they are in a scene.
+*/
+void TextResizeHandles::installHandleFilters()
+{
+	if (!scene())
+		return;
+	for (QetGraphicsHandlerItem *handle : std::as_const(m_handles))
+		if (handle->scene() == scene())
+			handle->installSceneEventFilter(this);
+}
+
 bool TextResizeHandles::sceneEventFilter(QGraphicsItem *watched, QEvent *event)
 {
 	const int index = m_handles.indexOf(static_cast<QetGraphicsHandlerItem *>(watched));

sources/QetGraphicsItemModeler/textresizehandles.h

@@ -65,8 +65,10 @@ class TextResizeHandles : public QGraphicsObject
 
 	protected:
 		bool sceneEventFilter(QGraphicsItem *watched, QEvent *event) override;
+		QVariant itemChange(GraphicsItemChange change, const QVariant &value) override;
 
 	private:
+		void installHandleFilters();
 		void pressed(TextResize::Corner corner, const QPointF &scene_pos);
 		void moved(const QPointF &scene_pos);
 		void released();

tests/qttest/tst_textresizehandles.cpp

@@ -215,6 +215,38 @@ private slots:
 			}
 	}
 
+	// The handles still work after the text left its scene and came back,
+	// as undoing a delete does: Qt drops scene event filters on removal.
+	void dragAfterLeavingTheScene()
+	{
+		QGraphicsScene scene;
+		auto *text = new WidthText(QStringLiteral("Motor protection switch Q12"));
+		scene.addItem(text);
+		auto *handles = new TextResizeHandles(text);
+
+		scene.removeItem(text);
+		scene.addItem(text);
+
+		const QPointF start_pos = text->pos();
+		drag(scene, *handles, *text, TextResize::BottomRight, QPointF(-40, 0));
+		QVERIFY(text->textWidth() > 0);
+		QCOMPARE(text->pos(), start_pos);
+		delete text;
+	}
+
+	// The handles also work when they are created before the text is in a
+	// scene.
+	void dragWhenCreatedOutsideAScene()
+	{
+		QGraphicsScene scene;
+		auto *text = new WidthText(QStringLiteral("Motor protection switch Q12"));
+		auto *handles = new TextResizeHandles(text);
+		scene.addItem(text);
+
+		drag(scene, *handles, *text, TextResize::BottomRight, QPointF(-40, 0));
+		QVERIFY(text->textWidth() > 0);
+	}
+
 	// A text is never made narrower than its longest word.
 	void dragStopsAtTheLongestWord()
 	{