Mouse: click a body to select it. Drag from empty space to launch a new body — the drag vector is its velocity and the dotted line previews its path (integrated forward in the live field). Right-drag orbits, Shift+right-drag or middle pans, wheel zooms. Touch: one finger launches, two fingers orbit/zoom (or enable “Left-drag orbits camera”).
Reverse runs the leapfrog integrator with negative dt, so orbits retrace exactly until a merge or absorption (irreversible) happens. Energy drift is measured on the gravitating bodies and is rebased whenever bodies are added, removed or merged.