htmlvideoelement: Fix poster frame processing algorithm (#37533)

According to HTML specification the poster attribute determines the
element's poster frame
(regardless of the value of the element's show poster flag).
https://html.spec.whatwg.org/multipage/#poster-frame

So the poster frame and the show poster flag is orthogonal to each
other, the latest one only controls
when browser should display the poster frame and should do not block
accepting video frames
from media pipeline (e.g. on new_preroll callback from video sink).

During layout the video element should select the current frame which
will be presented
based on first matching condition from the list:

https://html.spec.whatwg.org/multipage/#the-video-element:the-video-element-7

Testing: Improvements in the following WPT tests
- html/canvas/element/manual/imagebitmap/createImageBitmap*
- html/semantics/embedded-content/the-canvas-element/*

Fixes: #37165

---------

Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/htmlvideoelement.rs

@@ -133,6 +133,9 @@ impl HTMLVideoElement {
         sent_resize
     }
 
+    /// Gets the copy of the video frame at the current playback position,
+    /// if that is available, or else (e.g. when the video is seeking or buffering)
+    /// its previous appearance, if any.
     pub(crate) fn get_current_frame_data(&self) -> Option<Snapshot> {
         let frame = self.htmlmediaelement.get_current_frame();
         if frame.is_some() {
@@ -163,22 +166,31 @@ impl HTMLVideoElement {
     }
 
     /// <https://html.spec.whatwg.org/multipage/#poster-frame>
-    fn fetch_poster_frame(&self, poster_url: &str, can_gc: CanGc) {
-        // Step 1.
+    fn update_poster_frame(&self, poster_url: Option<&str>, can_gc: CanGc) {
+        // Step 1. If there is an existing instance of this algorithm running
+        // for this video element, abort that instance of this algorithm without
+        // changing the poster frame.
         self.generation_id.set(self.generation_id.get() + 1);
 
-        // Step 2.
-        if poster_url.is_empty() {
+        // Step 2. If the poster attribute's value is the empty string or
+        // if the attribute is absent, then there is no poster frame; return.
+        if poster_url.is_none_or(|url| url.is_empty()) {
+            self.htmlmediaelement.set_poster_frame(None);
             return;
         }
 
-        // Step 3.
-        let poster_url = match self.owner_document().url().join(poster_url) {
+        // Step 3. Let url be the result of encoding-parsing a URL given
+        // the poster attribute's value, relative to the element's node
+        // document.
+        // Step 4. If url is failure, then return. There is no poster frame.
+        let poster_url = match self.owner_document().url().join(poster_url.unwrap()) {
             Ok(url) => url,
-            Err(_) => return,
+            Err(_) => {
+                self.htmlmediaelement.set_poster_frame(None);
+                return;
+            },
         };
 
-        // Step 4.
         // We use the image cache for poster frames so we save as much
         // network activity as possible.
         let window = self.owner_window();
@@ -228,7 +240,9 @@ impl HTMLVideoElement {
 
     /// <https://html.spec.whatwg.org/multipage/#poster-frame>
     fn do_fetch_poster_frame(&self, poster_url: ServoUrl, id: PendingImageId, can_gc: CanGc) {
-        // Continuation of step 4.
+        // Step 5. Let request be a new request whose URL is url, client is the element's node
+        // document's relevant settings object, destination is "image", initiator type is "video",
+        // credentials mode is "include", and whose use-URL-credentials flag is set.
         let document = self.owner_document();
         let request = RequestBuilder::new(
             Some(document.webview_id()),
@@ -243,7 +257,8 @@ impl HTMLVideoElement {
         .insecure_requests_policy(document.insecure_requests_policy())
         .has_trustworthy_ancestor_origin(document.has_trustworthy_ancestor_origin())
         .policy_container(document.policy_container().to_owned());
-        // Step 5.
+
+        // Step 6. Fetch request. This must delay the load event of the element's node document.
         // This delay must be independent from the ones created by HTMLMediaElement during
         // its media load algorithm, otherwise a code like
         // <video poster="poster.png"></video>
@@ -264,12 +279,15 @@ impl HTMLVideoElement {
         self.generation_id.get()
     }
 
+    /// <https://html.spec.whatwg.org/multipage/#poster-frame>
     fn process_image_response(&self, response: ImageResponse, can_gc: CanGc) {
+        // Step 7. If an image is thus obtained, the poster frame is that image.
+        // Otherwise, there is no poster frame.
         match response {
             ImageResponse::Loaded(image, url) => {
                 debug!("Loaded poster image for video element: {:?}", url);
                 match image.as_raster_image() {
-                    Some(image) => self.htmlmediaelement.process_poster_image_loaded(image),
+                    Some(image) => self.htmlmediaelement.set_poster_frame(Some(image)),
                     None => warn!("Vector images are not yet supported in video poster"),
                 }
                 LoadBlocker::terminate(&self.load_blocker, can_gc);
@@ -277,6 +295,7 @@ impl HTMLVideoElement {
             ImageResponse::MetadataLoaded(..) => {},
             // The image cache may have loaded a placeholder for an invalid poster url
             ImageResponse::PlaceholderLoaded(..) | ImageResponse::None => {
+                self.htmlmediaelement.set_poster_frame(None);
                 // A failed load should unblock the document load.
                 LoadBlocker::terminate(&self.load_blocker, can_gc);
             },
@@ -340,10 +359,9 @@ impl VirtualMethods for HTMLVideoElement {
 
         if attr.local_name() == &local_name!("poster") {
             if let Some(new_value) = mutation.new_value(attr) {
-                self.fetch_poster_frame(&new_value, CanGc::note())
+                self.update_poster_frame(Some(&new_value), CanGc::note())
             } else {
-                self.htmlmediaelement.clear_current_frame_data();
-                self.htmlmediaelement.set_show_poster(false);
+                self.update_poster_frame(None, CanGc::note())
             }
         };
     }
@@ -521,7 +539,7 @@ impl LayoutHTMLVideoElementHelpers for LayoutDom<'_, HTMLVideoElement> {
         let video = self.unsafe_get();
 
         // Get the current frame being rendered.
-        let current_frame = video.htmlmediaelement.get_current_frame_data();
+        let current_frame = video.htmlmediaelement.get_current_frame_to_present();
 
         // This value represents the natural width and height of the video.
         // It may exist even if there is no current frame (for example, after the