Look at the desktop edges first
Overscan hides the margins of the image beyond the panel’s visible edges. If the Mac menu bar or window corners are cut off, picture fit is a useful question. Black bars around a film are different: they may preserve the programme’s intended shape rather than indicate lost picture.
Use a desktop or reference with visible corners to separate the two observations. If the entire desktop fits while only one film has bars, avoid enlarging every input merely to remove that film’s original aspect ratio.
Select the television in macOS
Apple directs users to System Settings, Displays, then the television’s display settings. An Underscan adjustment may be available for the TV. Make sure you are changing the external television rather than the laptop panel.
Use small adjustments and inspect all four corners. The aim is to show the complete desktop at the intended shape. A picture that reaches every edge by cutting off labels does not complete that task.
Use the TV controls when needed
If macOS does not expose Underscan for the connected TV, Apple recommends the television’s own fitting settings. Names vary: Screen Fit, aspect ratio, zoom or computer mode may be relevant. Use the manual for the exact model and input.
After changing the TV, recheck the Mac view before adjusting another layer. Combining television zoom, macOS underscan and an app’s crop mode can conceal which correction is responsible. Retain the setting that solves the actual edge loss.
Keep film shape and interface fit separate
Play a familiar item after the desktop fits. If bars remain only on a wider film, judge whether the full image is visible rather than demanding that every source fill the panel. Stretching can make faces and text appear wider without recovering any missing detail.
Record whether the problem affects system menus, all video or one title. That scope guides the next action and prevents a global television change from being used to solve a single app’s picture-sizing preference.
Questions answered
Are all black bars an underscan fault?
No. Bars can preserve a programme’s aspect ratio; check whether the desktop edges also fail to fit.
Why is there no Underscan slider on my Mac?
Apple says availability depends on the connected TV. Use its documented picture-fit controls when the slider is absent.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.