+ fix possible inconsistency of arrow button in task header

src/Gui/QSint/actionpanel/actiongroup.cpp

@@ -143,6 +143,7 @@ void ActionGroup::processHide()
   if (!--m_foldStep) {
     myDummy->setFixedHeight(0);
     myDummy->hide();
+    myHeader->setFold(false);
     setFixedHeight(myHeader->height());
     setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred);
     return;
@@ -165,6 +166,7 @@ void ActionGroup::processShow()
     myDummy->hide();
     m_foldPixmap = QPixmap();
     myGroup->show();
+    myHeader->setFold(true);
     setFixedHeight(m_fullHeight+myHeader->height());
     setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred);
     setMaximumHeight(QWIDGETSIZE_MAX);

src/Gui/QSint/actionpanel/taskheader_p.cpp

@@ -196,7 +196,9 @@ void TaskHeader::fold()
 {
   if (myExpandable) {
     emit activated();
-
+    // Toggling the 'm_fold' member here may lead to inconsistencies with its ActionGroup.
+    // Thus, the method setFold() was added and called from ActionGroup when required.
+#if 0
     m_fold = !m_fold;
     changeIcons();
     if (myButton) {
@@ -207,6 +209,23 @@ void TaskHeader::fold()
         myButton->update();
       }
     }
+#endif
+  }
+}
+
+void TaskHeader::setFold(bool on)
+{
+  if (myExpandable) {
+    m_fold = on;
+    changeIcons();
+    if (myButton) {
+      myButton->setProperty("fold", m_fold);
+      if (myButton->style()) {
+        myButton->style()->unpolish(myButton);
+        myButton->style()->polish(myButton);
+        myButton->update();
+      }
+    }
   }
 }
 

src/Gui/QSint/actionpanel/taskheader_p.h

@@ -37,6 +37,9 @@ public:
 Q_SIGNALS:
   void activated();
 
+public:
+  void setFold(bool);
+
 public Q_SLOTS:
   void fold();