This was quite sudden: I have a Firefox tab showing a topic in the Software & Applications forum - it was minimized and I expanded it to Unminimized - it zoomed across the screen ending by showing a white line about 2 mm deep at the very top of the screen, and of the width of the unminimized window. I have found that I can drag its bottom boundary to show most of the window, but the top is inaccessible.
Now however the top of that window has appeared in this workspace, which is directly above the one where it was opened!
Throughout this episode it has always appeared quite happily maximized, and now i have just set it as maximized it its own workspace (below this one) and it has disappeared from this.
Restored to Unmaximised, again it overlaps both workspaces!
Over to the developers - with the best of luck!