So I decided to write an extension out of it. You can get it from here.
It does following things:
- Pauses the video at the start, i.e. disables the autoplay
- It calculates an estimated time it would take to download the video completely. Shows it in a red box in the top left corner.
- It also prepends a short version of this estimated time to the title of page. So even if you are browsing in other tabs or windows, you can tell how much longer it is going to take, without visiting the youtube page.
- There is an instant of time, when the video is not yet downloaded completely, but if you start watching the video after that point, you can be assured that the entire video will be downloaded by the time you reach the end. That way it is guaranteed that you won't have any interruption while watching the video. The extension smartly calculates that instant and informs you by turning the red box into orange. It also adds '*' to the timestamp in title after this point.
Give it a shot. The extension won't yet work with HTML5 player, I will fix that soon though.