Chrome Version: 64.0.3280.5
OS: Chromebook Pixelbook

What steps will reproduce the problem?
(1) Visit https://www.youtube.com/watch?v=1La4QzGeaaQ
(2) Select 4K
(3) Click frenetically on play/pause YouTube player button until video currentTime is more than 7 seconds
(4) Check out Media Capabilities decoding info

What is the expected result?
The 4K VP9 video playback should be marked as supported, smooth and power efficient (as this is what happens usually)

What happens instead?

Here's what I get from https://beaufortfrancois.github.io/sandbox/media-capabilities/decoding-info-2.html:

VIDEO/WEBM; CODECS=VP9
----------------------

NOT SMOOTH               | 3840x2160 (4K) | 4000 kbps | 60 fps | file        
NOT POWER EFFICIENT      | 3840x2160 (4K) | 4000 kbps | 60 fps | file        
NOT SMOOTH               | 3840x2160 (4K) | 4000 kbps | 60 fps | media-source
NOT POWER EFFICIENT      | 3840x2160 (4K) | 4000 kbps | 60 fps | media-source
NOT SMOOTH               | 3840x2160 (4K) | 2500 kbps | 60 fps | file        
NOT POWER EFFICIENT      | 3840x2160 (4K) | 2500 kbps | 60 fps | file        
NOT SMOOTH               | 3840x2160 (4K) | 2500 kbps | 60 fps | media-source
NOT POWER EFFICIENT      | 3840x2160 (4K) | 2500 kbps | 60 fps | media-source
NOT SMOOTH               | 3840x2160 (4K) | 1500 kbps | 60 fps | file        
NOT POWER EFFICIENT      | 3840x2160 (4K) | 1500 kbps | 60 fps | file        
NOT SMOOTH               | 3840x2160 (4K) | 1500 kbps | 60 fps | media-source
NOT POWER EFFICIENT      | 3840x2160 (4K) | 1500 kbps | 60 fps | media-source
NOT SMOOTH               | 3840x2160 (4K) |  700 kbps | 60 fps | file        
NOT POWER EFFICIENT      | 3840x2160 (4K) |  700 kbps | 60 fps | file        
NOT SMOOTH               | 3840x2160 (4K) |  700 kbps | 60 fps | media-source
NOT POWER EFFICIENT      | 3840x2160 (4K) |  700 kbps | 60 fps | media-source



And here are the raw results with https://github.com/beaufortfrancois/video-decode-stats:

$ node main.js 
12|3840x2160|60  :  DecodeStatsProto {
  framesDecoded: Long { low: 467, high: 0, unsigned: true },
  framesDropped: Long { low: 76, high: 0, unsigned: true },
  framesDecodedPowerEfficient: Long { low: 0, high: 0, unsigned: true } }