Same geometry and same clipping plane. Only the viewport viewing angle changes. In one direction, clipped edges extend beyond the section boundary; from the opposite direction, they are displayed correctly.
Position 1 shows the artifacts; Position 2 shows the correct display.
The issue occurs with hardware OpenGL on an NVIDIA GeForce RTX 4060 Laptop GPU and is reproducible with both NVIDIA 610.88 and the older Acer OEM 546.32 driver.
With hardware OpenGL, the clipping cap is displayed, but clipped edges extend beyond the section boundary depending on the viewport viewing direction.
With Software OpenGL, the extended edges are no longer visible, but the clipping cap surface itself is missing, so the result is still incorrect. In Software OpenGL the viewing direction does not seem to make a difference.
Disabling GPU Tessellation has no effect.
Lowering the OpenGL level to 2.1 makes the extended-edge artifacts disappear while keeping the clipping cap visible.
Update: I found an important detail after further testing.
After completely uninstalling VisualARQ, the clipping artifacts disappear. The same native Rhino geometry and the same ClippingPlane are displayed correctly.
Simply disabling VisualARQ in Plug-in Manager and restarting Rhino was not enough — the issue was still present.
So this does not appear to be a native Rhino ClippingPlane/OpenGL issue as I initially suspected. It seems to be triggered by VisualARQ or one of its installed display components, even when clipping native Rhino geometry.