It can probably be made a bit simpler than this, but here’s a quick example of an animating canvas with a pause button:
https://beta.observablehq.com/@jashkenas/simple-canvas-animation-with-pause
It uses this, which is the previous value of the cell (in this case, the canvas), to keep the position of the circle the same even if the code has been re-evaluated. For more, see: https://beta.observablehq.com/@tmcw/untitled/5