Hide action panel when displaying a stage with a background

This is not the worst hack ever, but it's quite a hack.

Contributes to issue CURA-6054.

plugins/MonitorStage/MonitorMain.qml

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Ultimaker B.V.
+// Copyright (c) 2018 Ultimaker B.V.
+// Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.10
 import QtQuick.Controls 1.4
@@ -7,31 +8,27 @@ import UM 1.3 as UM
 import Cura 1.0 as Cura
 
 
-Item
+// We show a nice overlay on the 3D viewer when the current output device has no monitor view
+Rectangle
 {
-    // We show a nice overlay on the 3D viewer when the current output device has no monitor view
-    Rectangle
+    id: viewportOverlay
+
+    color: UM.Theme.getColor("viewport_overlay")
+    anchors.fill: parent
+
+    // This mouse area is to prevent mouse clicks to be passed onto the scene.
+    MouseArea
     {
-        id: viewportOverlay
-
-        color: UM.Theme.getColor("viewport_overlay")
         anchors.fill: parent
-
-        // This mouse area is to prevent mouse clicks to be passed onto the scene.
-        MouseArea
-        {
-            anchors.fill: parent
-            acceptedButtons: Qt.AllButtons
-            onWheel: wheel.accepted = true
-        }
-
-        // Disable dropping files into Cura when the monitor page is active
-        DropArea
-        {
-            anchors.fill: parent
-        }
+        acceptedButtons: Qt.AllButtons
+        onWheel: wheel.accepted = true
     }
 
+    // Disable dropping files into Cura when the monitor page is active
+    DropArea
+    {
+        anchors.fill: parent
+    }
     Loader
     {
         id: monitorViewComponent
@@ -45,4 +42,4 @@ Item
 
         sourceComponent: Cura.MachineManager.printerOutputDevices.length > 0 ? Cura.MachineManager.printerOutputDevices[0].monitorItem : null
     }
-}
+}

resources/qml/Cura.qml

@@ -252,7 +252,21 @@ UM.MainWindow
                 anchors.bottom: parent.bottom
                 anchors.rightMargin: UM.Theme.getSize("thick_margin").width
                 anchors.bottomMargin: UM.Theme.getSize("thick_margin").height
-                visible: CuraApplication.platformActivity
+
+                /*
+                Show this panel only if there is something on the build plate, and there is NOT an opaque item in front of the build plate.
+                This cannot be solved by Z indexing! If you want to try solving this, please increase this counter when you're done:
+                Number of people having tried to fix this by z-indexing: 2
+                The problem arises from the following render order requirements:
+                - The stage menu must be rendered above the stage main.
+                - The stage main must be rendered above the action panel (because the monitor page must be rendered above the action panel).
+                - The action panel must be rendered above the expandable components drop-down.
+                However since the expandable components drop-downs are child elements of the stage menu,
+                they can't be rendered lower than elements that are lower than the stage menu.
+                Therefore we opted to forego the second requirement and hide the action panel instead when something obscures it (except the expandable components).
+                We assume that QQuickRectangles are always opaque and any other item is not.
+                */
+                visible: CuraApplication.platformActivity && (main.item == null || !qmlTypeOf(main.item, "QQuickRectangle"))
             }
 
             Loader
@@ -818,4 +832,21 @@ UM.MainWindow
             }
         }
     }
+
+    /**
+     * Function to check whether a QML object has a certain type.
+     * Taken from StackOverflow: https://stackoverflow.com/a/28384228 and
+     * adapted to our code style.
+     * Licensed under CC BY-SA 3.0.
+     * \param obj The QtObject to get the name of.
+     * \param class_name (str) The name of the class to check against. Has to be
+     * the QtObject class name, not the QML entity name.
+     */
+    function qmlTypeOf(obj, class_name)
+    {
+        //className plus "(" is the class instance without modification.
+        //className plus "_QML" is the class instance with user-defined properties.
+        var str = obj.toString();
+        return str.indexOf(class_name + "(") == 0 || str.indexOf(class_name + "_QML") == 0;
+    }
 }