You need a test account before reproducing.
1. Go to http://www.nicovideo.jp/watch/sm9
2. Login with the button that reads ログイン画面へ (need some credentials)
3. Click on play
4. Click on one of the tag above the player
5. The player will move and shrink down to a small area in the top left corner and continue to play while you browse for new content.
6. Click on 画面サイズを大きくする
7. The player will move and expand back to its original position and continue to play.
7. The layout is completely broken and the player is no where to be seen.
Incriminating change was rolled out in http://trac.webkit.org/changeset/145296.