Navigate a lazy-loaded turbo-frame

Problem
---

Consider a `<turbo-frame>` element with `loading="lazy"`. Once it
becomes visible, the `AppearanceObserver` will delegate to the
`FrameController` to load its contents.

However, once visible, subsequent navigations of that frame will
continue to be deferred, since its `[loading]` attribute is still
deferred, in spite of the fact that the element is visible on that page.

Proposed change
---

This commit proposes to immediately navigate a `<turbo-frame>` element
with `[loading="eager"]` _or_ a `<turbo-frame>` element that has been
loaded previously.

Possible alternatives
---

As an alternative to adding another conditional to the guard clause,
could it be worthwhile to treat navigational changes driven by `<a>`
clicks or `<form>` submissions different from programatically modifying
the element's `[src]` attribute? Do Custom Elements support changing an
attribute's value without also kicking off change callbacks?

src/core/frames/frame_controller.ts

@@ -45,7 +45,7 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   }
 
   sourceURLChanged() {
-    if (this.loadingStyle == FrameLoadingStyle.eager) {
+    if (this.loadingStyle == FrameLoadingStyle.eager || this.hasBeenLoaded) {
       this.loadSourceURL()
     }
   }
@@ -272,6 +272,10 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
     return this.formSubmission !== undefined || this.loadingURL !== undefined
   }
 
+  get hasBeenLoaded() {
+    return !!this.element.loaded
+  }
+
   get isActive() {
     return this.element.isActive
   }

src/tests/fixtures/frames/hello.html

@@ -10,6 +10,8 @@
     <h1>Form</h1>
     <turbo-frame id="hello">
       <h2>Hello from a frame</h2>
+
+      <a href="/src/tests/fixtures/frames.html">Navigate #hello</a>
     </turbo-frame>
   </body>
 </html>

src/tests/functional/loading_tests.ts

@@ -37,6 +37,20 @@ export class LoadingTests extends TurboDriveTestCase {
     this.assert.equal(await contents.getVisibleText(), "Hello from a frame")
   }
 
+  async "test navigating a visible frame with loading=lazy navigates"() {
+    await this.clickSelector("#loading-lazy summary")
+    await this.nextBeat
+
+    const initialContents = await this.querySelector("#hello h2")
+    this.assert.equal(await initialContents.getVisibleText(), "Hello from a frame")
+
+    await this.clickSelector("#hello a")
+    await this.nextBeat
+
+    const navigatedContents = await this.querySelector("#hello h2")
+    this.assert.equal(await navigatedContents.getVisibleText(), "Frames: #hello")
+  }
+
   async "test changing src attribute on a frame with loading=lazy defers navigation"() {
     const frameContents = "#loading-lazy turbo-frame h2"
     await this.nextBeat