I have been looking at the Browser "Preview" pane with video files and have been looking at the Settings to these.
I think that the 'show in preview' tickbox for Video files should mean to only show first frame in the "preview" pane and should not mean auto-playing video:
In many supported file-managers such as XYplorer, there is a clear setting named 'AutoPlay' for this very purpose:
So I believe for XnView MP there should be a separate (combobox) setting in Settings>File list>file list for 'AutoPlay' to choose if a video or audio automatically begins playing or not in the "preview" pane.
The 'show in preview' tickbox setting in the 'Custom file list' should still remain but would now mean:
for Video = would just show first image frame
for Audio = in future display embedded Cover art or Cover art extracted from Internet/blank Cover Art
I think this would be more clearer
