Well, it's less bad.

src/components/AnimationProxy.tsx

@@ -0,0 +1,21 @@
+import React, { useEffect } from "react"
+
+import { FrameContext } from "./Canvas"
+
+interface AnimationProxyProps<T> {
+  things: T[],
+  factory: (thing: T) => JSX.Element
+}
+
+function AnimationProxy<T>({ things, factory }: AnimationProxyProps<T>) {
+  const frameCount = React.useContext(FrameContext)
+  const [keptThings, setKeptThings] = React.useState<T[]>([])
+
+  useEffect(() => {
+    setKeptThings(things)
+  }, [frameCount])
+
+  return <>{keptThings.map((thing) => factory(thing))}</>
+}
+
+export default AnimationProxy

src/components/Canvas.tsx

@@ -2,29 +2,48 @@ import React from "react"
 
 import CanvasContext from "./CanvasContext"
 
+export const FrameContext = React.createContext(0)
+
 export interface SystemCanvasProps {
   height: number,
   width: number,
-  children?: JSX.Element[]
+  children?: JSX.Element[] | JSX.Element
 }
 
 function SystemCanvas({ height, width, children }: SystemCanvasProps) {
   const canvasRef = React.useRef<HTMLCanvasElement>(null)
   const [context, setContext] = React.useState<CanvasRenderingContext2D>(null)
+  const [frameCount, setFrameCount] = React.useState(0)
 
   React.useEffect(() => {
-    const context = canvasRef.current.getContext("2d")
+    if (canvasRef.current != null) {
+      const context = canvasRef.current.getContext("2d")
+      setContext(context)
+    }
+  }, [frameCount])
+
+  React.useEffect(() => {
+    const frameId = requestAnimationFrame(() => {
+      setFrameCount(frameCount + 1)
+    })
+    return () => {
+      cancelAnimationFrame(frameId)
+    }
+  }, [frameCount])
+
+  if (context != null) {
     context.save()
     context.fillStyle = "black"
     context.fillRect(0, 0, width, height)
     context.restore()
-    setContext(context)
+  }
-  })
 
   return (<div>
     <CanvasContext.Provider value={ context }>
-      <canvas width={width} height={height} ref={canvasRef} style={{ width, height }} />
+      <FrameContext.Provider value={ frameCount }>
-      {children}
+        <canvas width={width} height={height} ref={canvasRef} style={{ width, height }} />
+        {children}
+      </FrameContext.Provider>
     </CanvasContext.Provider>
   </div>
   )

src/components/Home.tsx

@@ -1,6 +1,7 @@
 import React, { useEffect, useState } from "react"
 
 import { Body, Bounds, Message } from "@app/types"
+import AnimationProxy from "./AnimationProxy"
 import Canvas from "./Canvas"
 import Client from "@app/client"
 import RenderedBody from "./RenderedBody"
@@ -49,12 +50,12 @@ function Home() {
       <h1>Hello, World!</h1>
       <p>Iteration: { iteration }, message count: { messageCount }, nObjects: { bodies.length }.</p>
       <Canvas height={actualBounds.height} width={actualBounds.width}>
-        {bodies.map((body) => <RenderedBody
+        <AnimationProxy things={bodies} factory={(body) => <RenderedBody
           key={body.id}
           body={body}
           bounds={bounds}
           actualBounds={actualBounds}
-        />)}
+        />}/>
       </Canvas>
     </>
   )