How to Fix Flickering on the Dell S2721DGF
Causes the S2721DGF being driven below its native 165Hz refresh, Adaptive-Sync (FreeSync / G-Sync Compatible) fluctuation flicker near the bottom of the variable range, a loose or low-bandwidth DisplayPort/HDMI cable, an incompatible GPU driver, and a faulty USB-C/DP adapter chain. Fixes in order: set 165Hz, toggle Adaptive-Sync, swap the cable and port, update the GPU driver, and power-cycle.