OpenGL draw a partial object in a scroll bar

I am making a GUI in OpenGL (more specifically lwjgl). I tried very hard to explore different ways of doing this, but it's hard for me to find what I want. I don’t want to use any external libraries (only those built into OpenGL, even trying to avoid using GLUT), and I would like it to work on something that supports OpenGL (for example, frame buffer objects do not work on older graphics cards).

I am making a 3D graphical interface with a scroll bar as a component. The problem is that I don’t know how to draw a partial GUI component without having to do a lot of calculations to only display part of it. I make components from OpenGL primitives, not textures. I was hoping there was an easy way to do this, how to use multiple viewports. I don’t even understand what viewports are.

In short: I need to have a scrollable panel as a component that overlaps the other components of the graphical interface (since this will be a drop-down menu) and not allow any of the components of my panel to draw outside my panel.