Stylesheets: Fix empty line between header and content in QSInt

src/Gui/QSint/actionpanel/actiongroup.cpp

@@ -57,6 +57,7 @@ void ActionGroup::init(bool hasHeader)
     separator->setFrameShadow(QFrame::Raised);
     separator->setFixedHeight(separatorHeight);
     separator->setContentsMargins(8, 0, 8, 0);
+    separator->setProperty("class", "separator");
     layout->addWidget(separator);
 
     myGroup = new TaskGroup(this, hasHeader);

src/Gui/Stylesheets/FreeCAD.qss

@@ -1546,6 +1546,12 @@ QSint--ActionGroup QFrame[class="content"] QToolButton:checked {
   background-color: qlineargradient(x1:0, y1:0, x2:0, y2:1, stop:0 @ButtonTopBackgroundColor, stop:1 @ButtonBottomBackgroundColor );
 }
 
+/* hide the empty line between header and content */
+QSint--ActionGroup QFrame[class="separator"] {
+  min-height: 0;
+  max-height: 0;
+}
+
 /* QToolButtons with a menu found in Sketcher task panel*/
 QSint--ActionGroup QToolButton::menu-button {
   border-left: 1px solid @GeneralBorderColor;