The Hot Corner Nuisance
The GNOME desktop environment (the default for Ubuntu) relies heavily on the “Activities” overview to manage open windows and launch applications. To make accessing this overview faster, Ubuntu features a hidden “Hot Corner” located in the extreme top-left of the screen. If you push your mouse cursor firmly into that corner, the desktop automatically shrinks all your windows and brings up the search bar and application grid. While designed for speed, this is incredibly frustrating if you frequently use the “Back” button in a maximized web browser (which sits very close to that corner); a slight overshoot will violently pull you out of your workflow and into the Activities overview.
Disabling the Active Edge
To prevent accidental workspace switching and stop Ubuntu from automatically activating the Activities overview when your mouse hits the top-left corner, you must disable the Hot Corner feature.
Unfortunately, Ubuntu does not expose this setting in the standard Settings app; you need the official tweaking tool. Open your application launcher and search for GNOME Tweaks (install it via the terminal with sudo apt install gnome-tweaks if you haven’t already). Open the Tweaks application. In the left-hand navigation sidebar, click on Top Bar. In the right-hand panel, look for the toggle switch explicitly named Activities Overview Hot Corner. Click this toggle to turn it completely Off. The change is instantaneous. You can now throw your mouse cursor into the top-left corner as aggressively as you like, and your desktop will remain perfectly stable. You can still access the overview by pressing the Super (Windows) key.