6G will require on-device antenna systems to operate at ultra-high frequency bands, achieve robust beamforming on the compact user devices, and be blockage-robust. Conventional edge-mounted antennas on devices have limited apertures, suffer from the 'death grip' caused by user-induced blockage, and have poor scalability at mmWave and sub-THz bands. To address these issues, motivated by the rapid evolution of transparent materials and antennas, we propose ScreenAnt in this work--which integrates a transparent antenna array onto the screens of future mobile devices. Specifically, we propose using a transparent on-screen uniform planar array and develop a framework to model its electromagnetic property, spatial configuration, and blockage robustness under realistic user-induced blockage. We also design a gradient-ascent-based algorithm to efficiently optimize power and phase control of on-screen antennas to maximize ScreenAnt's spectral efficiency. Our thorough simulations show that the proposed ScreenAnt can increase the uplink spectral efficiency by over 50% compared to edge-mounted antennas at 28 GHz, and by more than 150% at 300 GHz. ScreenAnt also demonstrates strong robustness against user-induced blockage, paving the way for practical and high-capacity 6G user device designs.