QGIS allows overview extents to be drawn below map labels (the new default), but in 3.6 choices have been added for drawing them below all map layers, or above/below a specific map layer. This allows users to control exactly where in the map item's layer stack the overview will be placed, e.g. allowing them to draw a overview extent below some feature layers such as roads whilst drawing it above other background layers.
This feature was developed by Nyall Dawson (North Road)