Using the modifier
.environment(\.horizontalSizeClass, .compact)
does work but it disables NavigationSplitView code so, if you only have Stack Views in your app, it’s fine; however, it’s not a viable option for me. I’m losing half of an active Search Bar (including its Cancel Button) under the left hand edge of a Tab Bar in Portrait orientation. The Search Bar sits below the Navigation Bar in Portrait orientation, avoiding any conflict with the Tab Bar, until it is activated. I wish it stayed put!