Fix:

a) Project stayed in modified state even after undoing all changes.
b) Saving the project deleted the undo stack.

sources/projectview.cpp

@@ -726,7 +726,10 @@ QETResult ProjectView::doSave()
 	// write to file
 	QETResult result = m_project -> write();
 	updateWindowTitle();
-	project()->undoStack()->clear();
+	// This marks the stack's current index as the new "saved" point (so isClean()/cleanChanged() correctly
+	// resume tracking unsaved changes from here) without discarding the undo history. Edits made before this save 
+	// stay undoable for the rest of the session.
+	project()->undoStack()->setClean();
 	return(result);
 }
 

sources/qetdiagrameditor.cpp

@@ -2840,7 +2840,7 @@ void QETDiagramEditor::updateWindowModifiedState()
 		setWindowTitle(QString("%1[*] - %2").arg(
 			project->pathNameTitle(),
 			tr("QElectroTech", "window title")));
-		setWindowModified(project->projectOptionsWereModified());
+		setWindowModified(project->projectWasModified());
 	} else {
 		setWindowTitle(tr("QElectroTech", "window title"));
 		setWindowModified(false);

sources/qetproject.cpp

@@ -534,7 +534,9 @@ QString QETProject::pathNameTitle() const
 			)
 		).arg(final_title);
 	}
-	if (m_modified) {
+	// Same condition as projectWasModified(): project-options changeg (m_modified) OR the undo stack sitting away from
+	// its clean index. 
+	if (m_modified || !m_undo_stack->isClean()) {
 		final_title = QString(
 			tr(
 				"%1 [modifié]",

sources/qetproject.h

@@ -252,7 +252,26 @@ class QETProject : public QObject
 		void updateDiagramsTitleBlockTemplate(TitleBlockTemplatesCollection *, const QString &);
 		void removeDiagramsTitleBlockTemplate(TitleBlockTemplatesCollection *, const QString &);
 		void usedTitleBlockTemplateChanged(const QString &);
-		void undoStackChanged (bool a) {if (!a) setModified(true);}
+		/* Deliberately does NOT touch m_modified: m_modified /
+		 * setModified() track project-OPTIONS changes only (see
+		 * projectOptionsWereModified()), which have no undo
+		 * entry and so must stay set until an explicit write().
+		 * Diagram-content changes are tracked by the undo
+		 * stack's own clean index instead, and projectWasModified()
+		 * already ORs the two together -- that combined value is
+		 * what actually answers "does this project have unsaved
+		 * changes", so re-derive and broadcast it here on every
+		 * clean/dirty transition (covering, in particular, an
+		 * Undo that walks the stack back to its clean index).
+		 * Latching m_modified itself to the undo stack's dirty
+		 * state, the way this slot did before, is a one-way trap:
+		 * cleanChanged(true) would never come back through here
+		 * to un-set it, so a plain content edit stayed marked as
+		 * unsaved even after being fully undone. */
+		void undoStackChanged (bool /*a*/) {
+			emit projectModified(this, projectWasModified());
+			emit projectInformationsChanged(this);
+		}
 
 	private:
 		void readProjectXml(QDomDocument &xml_project);