Run the check
Show black, white, and primary-color patterns and note whether the line remains in exactly the same position.
Open the display's own menu over the line and check whether the mark passes through the menu as well.
Try a known-working cable and a second source, changing one connection at a time.
Read the result
A line visible over the monitor's own menu suggests a display-side issue; a line present only from one source points to the source or link. A photograph alone cannot distinguish those layers.
Limits and next step
The browser only renders patterns. It cannot inspect panel drivers, internal ribbon connections, or cable integrity.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.