The border has higher priority than pixels but lower priority than the black colour output when HSYNC/VSYNC are active.
While When border is active, all the CRTC counters continue to increment normally and addresses continue to be generated.
The DISPTMG signal is composited from its 3 internal subcomponents: HBORDER (Horizontal), VBORDER (Vertical), SBORDER (Split) by using the NOR function. DISPTMG is "0" if at least one of its subcomponents is "1". These 3 subcomponents are all independent of each other.