Quarter circle with self-filling edge when mouse pointer is near the corner in Big Sur

Question

I am new to Big Sur. While setting it up, I noticed a feature I had not seen before. Whenever the cursor got close to one of the four corners of the screen, a quarter-circle-like shape appeared adjusted to the specific corner and the edge of the quarter circle was gradually being filled up with color.

In the meantime, I have changed a lot of settings in Big Sur and I can no longer reproduce this phenomenon. As a result, I cannot post a screenshot/video and must rely on the verbal description above. If this description sounds familiar to anyone, I would be grateful for the identification of this feature. The closest match I could find is hot corners, but this quarter-circle thing definitely seems to be a distinct feature.

Answer 1

I figured it out. When you select Show Keyboard Viewer within the keyboard input menu in the menu bar, it automatically enables Accessibility Keyboard, in the same way as though you had checked Enable Accessibility Keyboard in System Preferences > Accessibility > Keyboard > Accessibility Keyboard.

Now, this feature has its own hot-corner functionality (the default setting is Hide / Show Home Panel in all four corners), but it only gets activated if the cursor keeps touching (or “dwelling on”) one of the corners for 1–2 seconds. After this delay, the keyboard viewer is hidden or shown once again. It is during this short delay that the quarter circle appears and its white edge gets gradually filled up with gray—see the picture below.