I managed to get a step further, the W3M recommendation to close the frame as soon as possible seems to be the key: in my case I was caching the N previous frames (to be able to play them backward for at least a few seconds) but it seems like in windows this freeze the decoder since apparently the resources to keep the frames are in control of the decoder. Now I need to see if I can find a way to support my backward-play feature… Either I need to spend some time to re-encode the video backward (it may take some time but is certainly the most robust option), or I find a way to move the cache to a part of the memory that is in control of the browser and not the decoder. But at least I know what I'm trying to avoid now!