I know this is a minor cosmetic request.....but when you have a negative global Z axis offset that is greater than one layer thickness, the preview is resolved with the first layer below the bed graphic...and it may be difficult to actually spot its there. This may lead to not being able to easily see for instance that the first layer is printing at a lower speed....because you can only see it if you tip the display so you're looking from underneath.
To make things easier, could either the preview be rendered ignoring the negative Z offset, so that its shown with ALL layers above the bed graphic, or for some visual indicator to advise that there are layers BELOW the bed graphic.