in dolphins information panel there is the ability to play music or videos. while audio only works good. videos are broken. when you mark a video and click on "play" it 

* sometimes will start but wont show progress and you can not stop it. when you now ciick on play again dolphin will open another windows an play the video in this window

* or shows a "black" preview but sound works.

im using AMDGPU ddx with Radeon R9 380. vaapi is enabled (i can watch all the videos without issue on "dragon" player wich should use the same engine as dolphin.

if the preview shows the "all black" preview it helps to resize the information panel.

Reproducible: Always

Steps to Reproduce:
1. open dolphin and go to a video directory
2. mark a video
3. click on "play" to get a preview

Actual Results:  
"sometimes" the video will be played, but you cannot stop it and it will open another window for only the videopreview if you try to press play again

the other times it only shows a black preview until you change the size of the information panel 

Expected Results:  
it should just show the videofile

output: while open dolphin > goto ~/videos/phone/ > mark a *.mp4 and click on play (black preview)

dolphin                                                                   :(
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