Vague details from memory: To make the vertical borders disappear, you could reset the refresh timing (maybe it was just by switching from 50Hz to 60Hz mode) ha

Vague details from memory: To make the vertical borders disappear, you could reset the refresh timing (maybe it was just by switching from 50Hz to 60Hz mode) halfway through a frame, and the video chip on the ST would desync from the retrace in the CRT. The only way I know to draw on the horizontal borders of a vanilla ST was by perfectly timed border color changes - I used that along with the compiled sprites technique discussed elsewhere in this post, to write a border-less horizontal scroller.