qml: fix anchoring of the layer effect in `MainDisplay`

This bug is currently asymptomatic because `stackViewParent`
and `mainRow` (its parent) have the same size.

We can anchor to the sibling like we do for visual parent,
so this is a trivial fix.

modules/gui/qt/maininterface/qml/MainDisplay.qml

@@ -278,7 +278,7 @@ FocusScope {
                     id: stackViewParentLayerEffect
 
                     // Setting `height` does not seem to work here. Anchoring the effect is not very nice, but it works:
-                    anchors.fill: parent
+                    anchors.fill: stackViewParent // WARNING: layered item is not necessarily the visual parent of its layer effect.
                     anchors.bottomMargin: -stackViewParent.bottomExtension
 
                     blending: stackViewParent.color.a < (1.0 - Number.EPSILON)