I have a simple Scaffold that when I put a topappbar on it it has some unexpected results. This first image shows how the test app looks before the topappbar. The second shows that when I add the topappbar multiple things (textview and the pseudo tab bar) disappear from the screen. I opened a defect with google which can be found here ->
https://issuetracker.google.com/u/1/issues/285560040. The source is also in the issue I opened with google