Add test coverage for turbo-frame[busy]

During the request lifecycle, `<turbo-frame>` elements will toggle the
`[busy]` boolean attribute to true when the request starts, and then
remove it when the request ends.

This commit adds functional test coverage for that behavior.

src/core/frames/frame_controller.ts

@@ -186,7 +186,8 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   // Form submission delegate
 
   formSubmissionStarted(formSubmission: FormSubmission) {
-
+    const frame = this.findFrameElement(formSubmission.formElement)
+    frame.setAttribute("busy", "")
   }
 
   formSubmissionSucceededWithResponse(formSubmission: FormSubmission, response: FetchResponse) {
@@ -203,7 +204,8 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
   }
 
   formSubmissionFinished(formSubmission: FormSubmission) {
-
+    const frame = this.findFrameElement(formSubmission.formElement)
+    frame.removeAttribute("busy")
   }
 
   // View delegate

src/tests/fixtures/form.html

@@ -115,10 +115,15 @@
     </div>
     <hr>
     <div id="targets-frame">
-      <form action="/__turbo/redirect" method="post" data-turbo-frame="frame">
+      <form action="/__turbo/redirect" method="post" data-turbo-frame="frame" class="one">
         <input type="hidden" name="path" value="/src/tests/fixtures/one.html">
         <button type="submit">Submit</button>
       </form>
+
+      <form action="/__turbo/redirect" method="post" data-turbo-frame="frame" class="frame">
+        <input type="hidden" name="path" value="/src/tests/fixtures/frames/frame.html">
+        <button type="submit">Submit</button>
+      </form>
     </div>
     <turbo-frame id="frame">
       <h2>Frame: Form</h2>

src/tests/functional/form_submission_tests.ts

@@ -185,6 +185,23 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.equal(await this.attributeForSelector("#frame", "src"), url.href, "redirects the target frame")
   }
 
+  async "test frame form submission toggles the ancestor frame's [busy] attribute"() {
+    await this.clickSelector("#frame form.redirect input[type=submit]")
+
+    this.assert.equal(await this.nextAttributeMutationNamed("frame", "busy"), "", "sets [busy] on the #frame")
+    this.assert.equal(await this.nextAttributeMutationNamed("frame", "busy"), null, "removes [busy] from the #frame")
+  }
+
+  async "test frame form submission toggles the target frame's [busy] attribute"() {
+    await this.clickSelector('#targets-frame form.frame [type="submit"]')
+
+    this.assert.equal(await this.nextAttributeMutationNamed("frame", "busy"), "", "sets [busy] on the #frame")
+
+    const title = await this.querySelector("#frame h2")
+    this.assert.equal(await title.getVisibleText(), "Frame: Loaded")
+    this.assert.equal(await this.nextAttributeMutationNamed("frame", "busy"), null, "removes [busy] from the #frame")
+  }
+
   async "test frame form submission with empty created response"() {
     const htmlBefore = await this.outerHTMLForSelector("#frame")
     const button = await this.querySelector("#frame form.created input[type=submit]")
@@ -300,7 +317,7 @@ export class FormSubmissionTests extends TurboDriveTestCase {
 
   async "test form submission targets disabled frame"() {
     this.remote.execute(() => document.getElementById("frame")?.setAttribute("disabled", ""))
-    await this.clickSelector('#targets-frame [type="submit"]')
+    await this.clickSelector('#targets-frame form.one [type="submit"]')
     await this.nextBody
 
     this.assert.equal(await this.pathname, "/src/tests/fixtures/one.html")

src/tests/functional/frame_tests.ts

@@ -25,6 +25,13 @@ export class FrameTests extends TurboDriveTestCase {
     this.assert.equal(otherEvents.length, 0, "no more events")
   }
 
+  async "test following a link driving a frame toggles the [busy] attribute"() {
+    await this.clickSelector("#hello a")
+
+    this.assert.equal(await this.nextAttributeMutationNamed("frame", "busy"), "", "sets [busy] on the #frame")
+    this.assert.equal(await this.nextAttributeMutationNamed("frame", "busy"), null, "removes [busy] from the #frame")
+  }
+
   async "test following a link to a page without a matching frame results in an empty frame"() {
     await this.clickSelector("#missing a")
     await this.nextBeat