Roll Edit is the operation to move the edit point between two clips on the timeline.
In Final Cut Pro by default this operation affects both the video and the audio portion of a clip.
In order to perform a roll edit only on the video portion of a clip you:
- Select both clips on the timeline.
- Expand audio — ctrl-S — or in the context menu
Now video and audio portion are shown separately on the main timeline
- Switch to the Trim tool — T — or click on the dropdown menu top left above the timeline.
- Now move the edit point between the video clips or audio clips freely. 🙂

Long video with more explanation and explicitly shown tool menu and context menu:
▶️ Final Cut Pro - Roll edit between two clips for audio and video portion together or separately