Thanks for your suggestions.
is there really that big difference between 1080p and 720p?
Yes, a video in 1280x720 is more blurry than a video in 1920x1080. If you watch a movie, you won't see a big difference, but when you play a 1st person adventure game, it is obvious.
When the player moves from A to B, the video makes the transition. It means that the first frame of the video is the same as scene A, and the last frame of the video is the same as scene B. As a result, when you play in HD 1080 with a sharp image, and suddenly see the videos in 720, it makes a difference.
webp has alpha / transparency support just like png
Ok, good to know. This is something I will probably check in the very end of the project, in order to convert all the PNG files to Webp with transparency.
So to come back to my problem, I have made a simple test:
I have taken a video in 720p that I have converted to VP8 (no sound) and MKV. I have saved the very first frame of this video to a Webp picture, and also the very last frame.
Then I have created a new empty project in VS, imported the first and last frames (the webp). I have created a Left click action on the first frame which plays the video and, at the very end, displays the last frame.
With this test scene, the result is the same... The video lags at the very very end, and the very last frames (let's say 1 or 2) are not played at all, which gives the impression of a sudden "jump" to the final webp picture.
Honestly I don't know if it is my computer or not. I didn't have this problem with VS 3 (except for the black frame issue).
Another test: the video will freeze if I try the same test with Xvid codec at the very end,a nd I have to press Escape in order to see the webp picture. I know that Xvid is not recommended, but in my opinion it proves that something happens with the player (on my PC or in VS, I don't know). Update:
I have just found the option ShowBlackScreenAfterVideoIsPlayed in the Explorer/eGame
If I activate it, of course the black screen is back (which I don't want), but the videos don't lag anymore. Weird!