Supplement:
This problem is related to what I mentioned previously (post). And it has been fixed after the Joplin v2.10.10 update but the same problem is now appearing when the search bar and "New Note" & "New Todo" buttons are separated into two rows.
@Dimitris I didn't find any custom CSS solution to fix the dead zone because the note list block height is defined by system calculations. So, you have to wait until Joplin fixed it.