Qml required property is not supported in Qt < 5.15

src/qml/ZoomBar.qml

@@ -13,9 +13,10 @@ Rectangle {
     SystemPalette { id: barPalette; colorGroup: SystemPalette.Disabled }
     property bool hoveredBar: barArea.containsMouse || barArea.pressed || zoomStart.isActive || zoomEnd.isActive
     property int barMinWidth: 1
-    required property real contentPos
-    required property real zoomFactor
-    required property bool fitsZoom
+    // TODO re-enable required keyword once we have Qt >=5.15 only
+    /*required*/ property real contentPos
+    /*required*/ property real zoomFactor
+    /*required*/ property bool fitsZoom
     property string toolTipText
     signal proposeContentPos(real proposedValue)
     signal proposeZoomFactor(real proposedValue)