Spotlight search bar is no longer centered, how do I put it back?
The answer is two-fold.
Centering it horizontally: Drag the spotlight bar horizontally and you will feel a slight stickiness to a place. That is where you have centered it horizontally.
Centering it vertically: Similar to above, drag the spotlight bar, vertically and you will a stickiness to a place, and that's where you reach the vertical center.
Same answer as https://apple.stackexchange.com/a/225746/364510 with additional image to describe. Also you can drag-drop search bar to reposition it as you want. Note: When you move to original location vertically or horizontally it will stick a bit and force feedback will happen from touchpad (click sound).