Before Big Sur, a hidden Dock (at the bottom of the screen) would be unhidden only if the mouse pointer was moved to the very bottom of the screen — you literally had to slam the mouse into that last row of pixels.
In Big Sur, there’s a wide “activation area”. The Dock unhides when the mouse pointer’s tail has only just disappeared off the bottom of the screen.
I spend a lot of time in JetBrains IDEs, which have a number of important controls in the status bar at the bottom. Working on Big Sur, the constant unwanted activation of the Dock has become a major irritation. My options are to work in a smaller window, giving up some screen real estate (but then why hide the Dock in the first place?) or to work in full screen mode (but I often need other applications open).
There are many lists of hidden Dock preferences available online, so I’m hoping that the width of this activation script might be configurable and not entirely hard-coded.
Edit: Using Apple's "Pixie" app from the "Additional Tools for Xcode" package, I've established that the Dock activates as soon as the point of the mouse cursor is in the bottom 20 rows of pixels.