The border has higher priority than pixels but lower priority than the black colour output when HSYNC/VSYNC are active.
The DISPTMG can be forced to 0 by using R8 (DISPTMG Skew) on type 0,3 and 4 or by setting R6=0 on type 1. It is not possible to force the DISPTMG on type 2.
== HSYNC and VSYNC ==