How do I rotate the viewport, without leaving the x/y-plane? I'm looking at an object, sometimes very close, to check visual heights. I'm in orthographic view, straight front.

Now I want to rotate slowly to the left/right side, without leaving the xy-plane (while constructing, not for animation). But every slightest move of the mouse kicks me into space, and I can't check exactly.
I know rotating while pressing alt key. But this is very crude snapping. I need to rotate smoothly or at least in about 5° steps.
I'd like to press Z key, and then keeping movement always stable in xy-plane (like on grap or rotate). It also could help on other axes, or even local coordinates.
How can this be done?
Updated based on the comment:

The option you look for is the "Orbit left" using the Numpad 4 and "Orbit Right" using Numpad 6 respectively. At least in my logic that is the only way to rotate the viewport(as in the title of the question) and is not leaving the xy-plane. Not sure how "Roll and Orbit is different here as the result seems to be the same.


The increase or decrease in angle is set default to 15 degree in the preferences here:


Setting the Rotation Angle to 1.0, would result in the "Orbiting" to be a lot smoother.

Here a little visual demonstration:


Happy Blending