fix playing event: only dispatch on paused-to-playing transition

Per MDN, playing fires "after playback is first started, and whenever
it is restarted." A second play() while already playing should be a
no-op. Both play and playing now only fire on the paused -> playing
transition.

src/browser/tests/element/html/media.html

@@ -64,21 +64,21 @@
     audio.play();
     testing.expectEqual('play,playing', events.join(','));
 
-    // Second play: already playing, fires only playing (not play)
+    // Second play: already playing, no events
     audio.play();
-    testing.expectEqual('play,playing,playing', events.join(','));
+    testing.expectEqual('play,playing', events.join(','));
 
     // Pause: playing -> paused, fires pause
     audio.pause();
-    testing.expectEqual('play,playing,playing,pause', events.join(','));
+    testing.expectEqual('play,playing,pause', events.join(','));
 
     // Second pause: already paused, no event
     audio.pause();
-    testing.expectEqual('play,playing,playing,pause', events.join(','));
+    testing.expectEqual('play,playing,pause', events.join(','));
 
     // Load: fires emptied
     audio.load();
-    testing.expectEqual('play,playing,playing,pause,emptied', events.join(','));
+    testing.expectEqual('play,playing,pause,emptied', events.join(','));
   }
 </script>
 

src/browser/webapi/element/html/Media.zig

@@ -144,8 +144,8 @@ pub fn play(self: *Media, page: *Page) !void {
     self._network_state = .NETWORK_IDLE;
     if (was_paused) {
         try self.dispatchEvent("play", page);
+        try self.dispatchEvent("playing", page);
     }
-    try self.dispatchEvent("playing", page);
 }
 
 pub fn pause(self: *Media, page: *Page) !void {