[twisty] Use IntersectionObserver to avoid rendering players until …

…they're on the screen.

Since we set `contain: size` on the `.wrapper` inside the player, this should not affect page layout.

Since the `TwistyProp` hierarchy can be
initialized and updated independent of the DOM,
this means that you can even call `.play()` on a
player before it starts rendering, and the
rendering will smoothly pick up fromq the middle of
the animation as soon as the player enters the
viewport.

(Note that there is no way to enforce any of this,
though, and it is entirely possible for future
code to make bad assumptions that breaks this. A
browser test would be very useful for this.)

src/cubing/twisty/views/TwistyPlayer.ts

@@ -16,14 +16,14 @@ import type { SetupToLocation } from "../model/props/puzzle/state/SetupAnchorPro
 import type { PuzzleID } from "../model/props/puzzle/structure/PuzzleIDRequestProp";
 import type { BackgroundThemeWithAuto } from "../model/props/viewer/BackgroundProp";
 import type { BackViewLayoutWithAuto } from "../model/props/viewer/BackViewProp";
-import {
-  type ControlPanelThemeWithAuto,
-  controlsLocations,
-} from "../model/props/viewer/ControlPanelProp";
 import type {
   ColorScheme,
   ColorSchemeWithAuto,
 } from "../model/props/viewer/ColorSchemeRequestProp";
+import {
+  type ControlPanelThemeWithAuto,
+  controlsLocations,
+} from "../model/props/viewer/ControlPanelProp";
 import type { ViewerLinkPageWithAuto } from "../model/props/viewer/ViewerLinkProp";
 import type { VisualizationFormatWithAuto } from "../model/props/viewer/VisualizationProp";
 import type { VisualizationStrategy } from "../model/props/viewer/VisualizationStrategyProp";
@@ -169,6 +169,22 @@ const propOnly: Record<string, boolean> = {
   experimentalMovePressCancelOptions: true,
 };
 
+let cachedSharedIntersectionObserver: IntersectionObserver | undefined;
+const intersectedCallback = Symbol("intersectedCallback");
+function waitForIntersection(player: TwistyPlayer) {
+  cachedSharedIntersectionObserver ??= new IntersectionObserver(
+    (entries, observer) => {
+      for (const entry of entries) {
+        if (entry.isIntersecting && entry.intersectionRect.height > 0) {
+          (entry.target as TwistyPlayer)[intersectedCallback]();
+          observer.unobserve(entry.target);
+        }
+      }
+    },
+  );
+  cachedSharedIntersectionObserver.observe(player);
+}
+
 /**
  * TwistyPlayer is the heart of `cubing.js`. It can be used to display a puzzle on a web page like this:
  *
@@ -232,11 +248,15 @@ export class TwistyPlayer
   #errorElem = document.createElement("div"); // TODO: Better pattern.
   #alreadyConnected = false; // TODO: support resetting
   async connectedCallback(): Promise<void> {
+    this.addCSS(twistyPlayerCSS);
+    waitForIntersection(this);
+  }
+
+  async [intersectedCallback](): Promise<void> {
     if (this.#alreadyConnected) {
       return;
     }
     this.#alreadyConnected = true;
-    this.addCSS(twistyPlayerCSS);
 
     this.addElement(this.#visualizationWrapperElem).classList.add(
       "visualization-wrapper",