I have the same problem but only in My Mac mini 2018, I also have BIAB latest installed in my 2014 Mac mini , both have Big Sur Latest ver. My 2014 Mac mini works fine no problem resizing window, I have the problem in my 2018 MM can't resize the window. The differences between the 2 is the internal graphics, T2 chip disable, both i5 processors difference Architectures. 2014 MM has 32" HP display , my 2018 has 2 displays 1 28" 4k HP and 1 24 " HP the only difference in the install was on my 2018 I install the real tracks on a ssd. My 2014 has everything installed on the main drive, It also could be any of the apps or plugins I have installed. I may install Big Sur in Parallels Desktop and then just install BIAB and see what happens. If it works then it's got to be another app or plugin thats causing it. I'll post and updates on how I make out.