The still image duration is greyed out and can’t set the length of a still image from the Properties tab, The “Set Default” button also seems to be not responding.
It is possible to adjust the image length on the timeline by dragging the handle of the image.
This is not a bug. Adjusting duration using the field is not permitted on the TIimeline - only when it is in the Source player. Use trimming to adjust duration in the Timeline.
