When editing a video in the Photos app, the Done button becomes unavailable after editing the start or end point of the video using the adjustable action in the VoiceOver rotor.
Select a video in the Photos app, then select the Edit button.
You should then be able to trim the start or end point of the video by placing VoiceOver focus on the Start or End element, then swiping up or down to change the value.
You should then be able to find the Done button and select it to complete the edit.
However, the Done button is dimmed, and therefore cannot be selected.
Bug First Encountered
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it is possible to place VoiceOver focus on the start or end element, and double-tap, hold, and drag the marker position.
This does make the Done button active, but allows much less control over the exact position of the marker, and therefore the accuracy of the edit.