I still haven’t gotten mine either.
In a recent preview a fix was made that causes the window to ignore the black bar when scaling the UI, but the black bar still exists in the latest preview (1.21.130.26).
What I’ve had to do is duplicate my display on my second monitor, then fullscreen the game, to trap the cursor on the game window. It is an annoying issue likely caused by 1.21.121’s cursor position fix.
/gamerule without parameters also caused my world to exit (error code Block) on Windows Preview 26.0.27.