If aa is False, you would need to glDisable whatever got enabled when aa is True.
I tried out this PR with #1910 and toggling AA from False->True->False would get a plot different from the initial plot.

@pijyoi Thank you for the hint, I haven't thought of that. I am no OpenGL-expert so I just reverted to the default-values from the OpenGL-documentation. Now this lets me toggle between aa-states in your (really nice) example.

In GLGridItem.py (l.68-72), there is a similar passage. Do you think it has the same problem then and should I fix it?

There are 3 other places that setup GL_LINE_SMOOTH: GLAxisItem, GLGridItem, GLLinePlotItem. GLAxisItem doesn't enable blending, but would probably get it enabled by GLGridItem. A proper fix might require all GLGraphicsItem(s) to disable everything that they have enabled before they exit their paint.

I would suggest to leave them as is. Don't modify things that we aren't able to test ourselves. Or raise a separate issue.

Probably after painting with antialiasing on, GL_LINE_SMOOTH should be disabled.
If we take a peek at: https://code.woboq.org/qt6/qtbase/src/opengl/qopenglpaintengine.cpp.html#_ZN28QOpenGL2PaintEngineExPrivate12resetGLStateEv
GL_LINE_SMOOTH happens to be one of the things that Qt library doesn't disable for us.
On the other hand, GL_BLEND is disabled for us, so we didn't actually need to disable it for non-antialiased mode.

Ok, so should I remove those lines again?

gl.glDisable(gl.GL_BLEND)
gl.glBlendFunc(gl.GL_ONE, gl.GL_ZERO)
gl.glHint(gl.GL_LINE_SMOOTH_HINT, gl.GL_DONT_CARE)

That seems to work too, but performance doesn't seem to be influenced by just removing the lines. Do you think we could introduce an attribute like _aa_state to PlotCurveItem which then calls the glEnable/glDisable-functions only when the state changes?

The problem is that when you have called beginNativePainting(), Qt makes little guarantee about the state. So things that we need, we need to set again for each frame. E.g. What would happen if we created 2 PlotWidgets in a single QMainWindow, one with antialiasing enabled and the other disabled? I would guess that because of the shared OpenGL context, they would see the last set setting of GL_LINE_SMOOTH.

So should I just remove the above lines to have a bit less to call when AA is disabled?

Yes. Based on my understanding, the only line needed would be

gl.glDisable(gl.GL_LINE_SMOOTH)

Btw, it's not so much for performance to reduce the number of lines here. Code like this tends to get copied around, so if we can, we keep the code to only the bits necessary.