Customization is purely a Compact Layout exercise. There is no separate Hover Detail editor in Lightning.
- Pick the object
Setup, Object Manager, click the object whose hover preview you want to change.
- Open Compact Layouts
Click Compact Layouts in the left nav. Find the Master layout or any custom one.
- Edit the field set
Edit the layout to include the four to six fields most useful in a quick preview (Name, Account, Phone, Email, Status, etc.).
- Save and verify
Save the layout. Hover over any record of this object in a list view; the popup should show the new field set.
- Test by user profile
Impersonate a restrictive-profile user and verify the popup still shows the expected fields. FLS can hide fields silently.
Gotchas
- Compact Layout changes propagate to multiple UI surfaces simultaneously. A change to support Hover Detail also changes the Highlights Panel, mobile view, and search results.
- Touch devices do not support hover. Designing a workflow that depends on Hover Detail leaves mobile users without that signal.
- Restrictive profiles can produce hover popups with missing fields. Test with the most-restricted profile to make sure the popup still communicates the essential info.
- Classic Mini Page Layouts do not automatically migrate to Compact Layouts when an org switches to Lightning. Verify the Compact Layout matches the old Mini Page Layout for objects users hover frequently.