You may have noticed that in recent Mac OS updates, Apple now allow for most program windows to be resized from all sides and corners, except seemingly from the top right corner which is reserved for making programs run in full-screen mode.
On any other edge of a program window, the mouse pointer changes shape to a double arrow, indicating you can resize the window. In the top right corner, the mouse pointer doesn’t change shape. Clicking once on the top right corner results in having the program run in full-screen mode.
However, if you click and hold on upper right corner, you can resize from that corner.