On all CRTCs, while a VSYNC is ongoing, the condition VCC=R7 is ignored. So we cannot trigger a new VSYNC during a VSYNC. Similarly, we cannot trigger a new HSYNC during an HSYNC.
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=== HSYNC ===
So even when a fully valid C-HSYNC pulse of 4µs is emitted, an HSYNC shorter than usual combined with a coloured border can induce a discolouration effect on those monitors.
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=== VSYNC ===