QTBUG-97828 described a regression revolving around the Qt6 Video element. When looping a video, there was a short, visible gap between the playbacks. The problem was fixed with Qt 6.3. 

However, the problem was fixed only for the case of looping the same video. However it still persists when switching to a different video. The transition used to be smooth in Qt 5.15 when setting "flushMode: VideoOutput.LastFrame".

See the following example (Qt 6.3.0)

import QtQuick
import QtMultimedia

Item {
    Video {
        source: "file:///home/root/video1.avi"
        width: 720
        height: 660

        onStopped: {
            source = "file:///home/root/video2.avi"
        }
    }
}

There is a short flicker in the transition between video1 and video2.

In Qt 5.15.3, you can make it work the following way:

import QtQuick 2.15
import QtMultimedia 5.15

Item {

    Video {
        source: "file:///home/root/video1.avi"
        autoPlay: true
        width: 720
        height: 660
        flushMode: VideoOutput.LastFrame

        onStopped: {
            source = "file:///home/root/video2.avi"
        }
    }
}

The flushMode property should be brought back into Qt6.