This happens to me all the time on the Mac, and it causes so much frustration and lost time.
My project window is separated, and then if the separator bar is pulled down to the bottom of the window accidentally, it cannot be grabbed and pulled back up! Your other tracks are down there, but you can’t see them or select them. When you try to grab the divider you instead grab the bottom of the entire window and it resizes. It can take forever to luck out and manage to grab the separator instead. The divider needs an actual handle that cannot disappear.
Mac. Nuendo ANY version.