Issue https://github.com/gjb2048/moodle-theme_essential/issues/415 has just recently been raised on Essential, but when I test it on Clean, the same thing happens. As a check I've tested that MDL-36499 has not caused a regression.
The problem being that on at least Chrome, the player height is 150px and get as soon as you start or use browser development tools, it sorts itself out. Any ideas as to what recent changes may have caused this. Replicate on Moodle 2.7.3+ (Build: 20141218) on Clean and Essential 188.8.131.52.3 (released soon). TDM off / on = no difference.