Created attachment 447942 [details]
Two simple test pages and a single mp4 video showcasing the bug
Certain videos on the web may start to jump back and forward between frames when the seek function of the video player is used.
This bug will suddenly be abcent when a second video element is placed on the page, it does not matter if this video element contains the same video or another video of a different format (eg. webm).
So far I have only noticed mp4 to be affected.
The bug is relevant for the latest stable and testing versions of Epiphany aswell as older versions. The bug is not a newly introduced regression, this bug has existed for a while already (6+ months at least), I do not recall when I first encountered it.
It seems to not be related to wrong encoding or something odd related to the video itself, as even when re-encoded with FFMPEG (ffmpeg -i video.mp4 video-out.mp4) the bug presists.
See the included attachment for two simple HTML pages and a video showcasing the bug. It will need to run on a webserver to display the videos properly.
I did not experience the bug on Safari thus I am assuming it to be WebkitGTK related.
Can you check which gst video decoder is being used? You can check the pipeline dumps, see https://github.com/Igalia/meta-webkit/wiki/Providing-useful-GStreamer-Zero-copy-issue-reports#in-webkit
I can reproduce this issue in Ephy TP (openh264dec) but not in Ephy Canary (gst-libav).
*** Bug 234725 has been marked as a duplicate of this bug. ***
I checked the Flatpak version of Epiphany, it was indeed using openh264dec.