I'm trying to connect a Dell 2208WFPT Flat Monitor to my MacBook Air (macOS Mojave, 10.14.6) using Thunderbolt. The symptoms are as follows : on the monitor, I only see the main background image and the upper menu bar of my Mac, and nothing else. In System Preferences > Display, the external monitor is not detected.
The specification for my MacBook Air says :
Video Support Simultaneously supports full native resolution on the built-in display and up to 3840-by-2160 resolution at 60Hz on an external display, both at millions of colors
Thunderbolt digital video output Native Mini DisplayPort output VGA, HDMI, DVI, and dual-link DVI output using adapters (sold separately)
I browsed https://support.apple.com/en-us/HT201177 and the related questions but found nothing useful so far.