SceneTickCallback typedef Widgets

Called once per frame with the total elapsed time and the deltaSeconds since the previous tick. Drive per-frame app logic here, or advance the scene with a supplied timestep via Scene.update (after which Scene.render skips its implicit wall-clock tick for that frame).

deltaSeconds measures wall time between ticks (see SceneView.clock), not the ticker's frame-begin timestamps. Those timestamps alternate between tiny and double-length deltas when GPU load pushes frames past a vsync even though frames present at an even cadence, and motion integrated against them staggers. elapsed stays timestamp-based (its accumulation carries no such noise), so the deltas can drift from it by a bounded frame's worth of skew.