LibWeb: Make the value of assignment to media currentTime the rhs value

In cases where a script assigns `x = video.currentTime = y`, we are
expected to have a result of `x === y`, even if the video's duration
is less than y.

According to the spec, this happens because the official playback
position is set to `y` in this case, but since we are following
implementations in making `currentTime` immediately return the position
on the valid media timeline, we have to specifically return the
unchanged value from the setter.

See: whatwg/html#11773

Author: Zaggy1024

Libraries/LibWeb/HTML/HTMLMediaElement.cpp

@@ -267,17 +267,23 @@ double HTMLMediaElement::current_time() const
 }
 
 // https://html.spec.whatwg.org/multipage/media.html#dom-media-currenttime
-void HTMLMediaElement::set_current_time(double current_time)
+double HTMLMediaElement::set_current_time(double current_time)
 {
     // On setting, if the media element's readyState is HAVE_NOTHING, then it must set the media element's default playback start
     // position to the new value; otherwise, it must set the official playback position to the new value and then seek to the new
     // value. The new value must be interpreted as being in seconds.
     if (m_ready_state == ReadyState::HaveNothing) {
         m_default_playback_start_position = current_time;
     } else {
-        m_official_playback_position = current_time;
+        // AD-HOC: Don't set the official playback position here, as seek_element() will set it according to the seekable
+        //         ranges. We return the value passed to the setter to ensure that chained assignments like
+        //             videoA.currentTime = videoB.currentTime = Number.MAX_VALUE;
+        //         will seek both videoA and videoB to their corresponding ending positions.
+        //         See https://github.com/whatwg/html/issues/11773
+
         seek_element(current_time);
     }
+    return current_time;
 }
 
 // https://html.spec.whatwg.org/multipage/media.html#dom-media-fastseek

Libraries/LibWeb/HTML/HTMLMediaElement.h

@@ -95,7 +95,7 @@ class HTMLMediaElement : public HTMLElement {
     WebIDL::ExceptionOr<void> load();
 
     double current_time() const;
-    void set_current_time(double);
+    double set_current_time(double);
     void fast_seek(double);
 
     double current_playback_position() const { return m_current_playback_position; }