Only snapshot frame visits that have actions

Frames that use `data-turbo-action` to promote their navigations to page
visits require cache snapshots to be taken prior to them loading, in
order for those snapshots to be accurate.

However snapshots can be expensive to create for complex pages. In cases
where a frame is *not* being promoted, we can avoid taking that
snapshot.

This helps with some performance issues observed when using frames on
very large pages.

Author: kevinmcconnell

src/core/frames/frame_controller.ts

@@ -64,7 +64,6 @@ export class FrameController
   readonly restorationIdentifier: string
   private previousFrameElement?: FrameElement
   private currentNavigationElement?: Element
-  pageSnapshot?: PageSnapshot
 
   constructor(element: FrameElement) {
     this.element = element
@@ -199,7 +198,6 @@ export class FrameController
   // Appearance observer delegate
 
   elementAppearedInViewport(element: FrameElement) {
-    this.pageSnapshot = PageSnapshot.fromElement(element).clone()
     this.proposeVisitIfNavigatedWithAction(element, element)
     this.loadSourceURL()
   }
@@ -369,7 +367,6 @@ export class FrameController
 
   private navigateFrame(element: Element, url: string, submitter?: HTMLElement) {
     const frame = this.findFrameElement(element, submitter)
-    this.pageSnapshot = PageSnapshot.fromElement(frame).clone()
 
     frame.delegate.proposeVisitIfNavigatedWithAction(frame, element, submitter)
 
@@ -382,6 +379,7 @@ export class FrameController
     this.action = getVisitAction(submitter, element, frame)
 
     if (this.action) {
+      const pageSnapshot = PageSnapshot.fromElement(frame).clone()
       const { visitCachedSnapshot } = frame.delegate
 
       frame.delegate.fetchResponseLoaded = (fetchResponse: FetchResponse) => {
@@ -395,7 +393,7 @@ export class FrameController
             willRender: false,
             updateHistory: false,
             restorationIdentifier: this.restorationIdentifier,
-            snapshot: this.pageSnapshot,
+            snapshot: pageSnapshot,
           }
 
           if (this.action) options.action = this.action