title: Why does item disappear when anchors changed?
body: <p>Here is a Tab in a TabView in my qml file:</p>
  
  <pre><code>    Tab {
              title: "Lab"
              Rectangle {
                  width: 300
                  height: 600
                  anchors.margins: 60
                  color: "gray"
  //                anchors.top: parent.top
  //                anchors.bottom: parent.verticalCenter
  //                anchors.margins: 10
                  Column {
                      id : buttons
  //                                    anchors.top: parent.top
  //                                    anchors.bottom: parent.verticalCenter
                      anchors.fill: parent
                                      anchors.margins: 10
                      spacing: 2
  
                      Repeater {
                          model: ["Add entity", "Add Unique Comp", "Add light", "Add Prefab", "Add entity to prefab", "None"]
  
                          Rectangle {
                              width: parent.width - 20
                              height: 24
                              anchors.horizontalCenter: parent.horizontalCenter
  
                              radius: 3
  
                              color: "lightBlue"
  
                              Button {
                                  width: parent.width * 0.5
                                  height: parent.height * 0.8
                                  anchors.left: parent.left
                                  anchors.verticalCenter: parent.verticalCenter
                                  text: index +": "+modelData
                                  onClicked: {
                                      console.log("clicked ", modelData);
                                  }
                              }
                          }
                      }
                  }
              }
          }
  </code></pre>
  
  <p>Here a TabView is used, above is one of the tabs. <code>x y</code> are not specified.</p>
  
  <p>If I remove <code>anchors.fill: parent</code>, the <code>Rectangle</code> will disappear on the screen. Why did this happen? Or how could I debug this?</p>