LinkPrefetchObserver: listen for complementary events

Prior to this commit, the `LinkPrefetchObserver` only listened for
`mouseleave` events to clear the `PrefetchCache` instance. Not only were
`mouseenter` events excluded, but the `mouseleave` event listeners were
attached directly to the `<a>` element with a `{ once: true }` option.

While unlikely, its was for those event listeners to never be removed if
a `mouseleave` were to not fire. Similarly, during `touchstart` events
the event listener were added, but never removed since there wasn't a a
complementary `mouseleave` event firing to remove it.

This commit makes two changes to the event listeners:

1. extract the `addEventListener` calls to a loop, looping over
   `mouseenter` and `touchstart` event names
2. define complementary events for both `mouseenter` and `touchstart`

By moving the cancellation logic out of individual event listeners and
into an `this.eventTarget`-wide scope, we limit the risk of leaking
listeners. Similarly, we only ever instantiate one per event-pairing.

To track the `<a>` element reference, define both a
`this.#tryToCancelPrefetchRequest` method and a `this.#linkToPrefetch`
property to hold the reference to the `<a>` element in question.

src/observers/link_prefetch_observer.js

@@ -9,10 +9,14 @@ import { StreamMessage } from "../core/streams/stream_message"
 import { FetchMethod, FetchRequest } from "../http/fetch_request"
 import { prefetchCache, cacheTtl } from "../core/drive/prefetch_cache"
 
+const observedEvents = {
+  "mouseenter": "mouseleave",
+  "touchstart": "touchcancel"
+}
+
 export class LinkPrefetchObserver {
   started = false
-  hoverTriggerEvent = "mouseenter"
-  touchTriggerEvent = "touchstart"
+  #linkToPrefetch = null
 
   constructor(delegate, eventTarget) {
     this.delegate = delegate
@@ -32,26 +36,30 @@ export class LinkPrefetchObserver {
   stop() {
     if (!this.started) return
 
-    this.eventTarget.removeEventListener(this.hoverTriggerEvent, this.#tryToPrefetchRequest, {
-      capture: true,
-      passive: true
-    })
-    this.eventTarget.removeEventListener(this.touchTriggerEvent, this.#tryToPrefetchRequest, {
-      capture: true,
-      passive: true
+    Object.entries(observedEvents).forEach(([startEventName, stopEventName]) => {
+      this.eventTarget.removeEventListener(startEventName, this.#tryToPrefetchRequest, {
+        capture: true,
+        passive: true
+      })
+      this.eventTarget.removeEventListener(stopEventName, this.#tryToCancelPrefetchRequest, {
+        capture: true,
+        passive: true
+      })
     })
     this.eventTarget.removeEventListener("turbo:before-fetch-request", this.#tryToUsePrefetchedRequest, true)
     this.started = false
   }
 
   #enable = () => {
-    this.eventTarget.addEventListener(this.hoverTriggerEvent, this.#tryToPrefetchRequest, {
-      capture: true,
-      passive: true
-    })
-    this.eventTarget.addEventListener(this.touchTriggerEvent, this.#tryToPrefetchRequest, {
-      capture: true,
-      passive: true
+    Object.entries(observedEvents).forEach(([startEventName, stopEventName]) => {
+      this.eventTarget.addEventListener(startEventName, this.#tryToPrefetchRequest, {
+        capture: true,
+        passive: true
+      })
+      this.eventTarget.addEventListener(stopEventName, this.#tryToCancelPrefetchRequest, {
+        capture: true,
+        passive: true
+      })
     })
     this.eventTarget.addEventListener("turbo:before-fetch-request", this.#tryToUsePrefetchedRequest, true)
     this.started = true
@@ -68,6 +76,7 @@ export class LinkPrefetchObserver {
       const location = getLocationForLink(link)
 
       if (this.delegate.canPrefetchRequestToLocation(link, location)) {
+        this.#linkToPrefetch = link
         const fetchRequest = new FetchRequest(
           this,
           FetchMethod.get,
@@ -77,12 +86,18 @@ export class LinkPrefetchObserver {
         )
 
         prefetchCache.setLater(location.toString(), fetchRequest, this.#cacheTtl)
-
-        link.addEventListener("mouseleave", () => prefetchCache.clear(), { once: true })
       }
     }
   }
 
+  #tryToCancelPrefetchRequest = (event) => {
+    if (event.target === this.#linkToPrefetch) {
+      prefetchCache.clear()
+    }
+
+    this.#linkToPrefetch = null
+  }
+
   #tryToUsePrefetchedRequest = (event) => {
     if (event.target.tagName !== "FORM" && event.detail.fetchOptions.method === "get") {
       const cached = prefetchCache.get(event.detail.url.toString())