Same forces, same timestep, three integrators — and then what a wobbly frame time does to all of them.
Scene Integrators uses the timestep slider. Fixed vs variable uses timestep + jitter + the toggle. Tunnelling uses speed + substeps + the swept toggle.
What to notice. Explicit Euler's energy line only ever climbs — the orbit spirals out, and no timestep small enough makes it stop, only slower.
Swapping two lines (semi-implicit) keeps energy wobbling about zero for free. Then turn jitter up: the variable-dt spring loses the reference and eventually detonates, while the fixed accumulator doesn't notice.