[FLT8]

I think it's probably about the timing. I'm not an expert, but I believe that opening the side borders requires cycle exact timing at both edges of the screen, and when you have characters enabled on the screen, the VIC-II graphics chip ends up 'stealing' memory cycles from the CPU every 8 scanlines to fetch character and font data from RAM. These lines are known as 'badlines'.

Across an entire scanline I think you normally get something like 63 6510 cpu cycles to do 'work' in, but only 23 if you hit a badline - keeping in mind that some instructions take multiple cycles to execute. This probably makes the timing difficult or impossible to manage with the characters turned on.

[phire]

It's not just that the 'badlines' steal 40 cycles. They steal a solid block of 40 cycles that cover most of the screen, from end of hblank until just before the right border starts. This blog post [1] has a nice interactive demo showing badline timings.

During a badline it's simply impossible write to the VIC-II's registers during the left border. Though, this seems to indicate it's still possible to open the right border during a bad line, but it's a 1 cycle window (maybe 2).

[1] https://nurpax.github.io/posts/2018-06-19-bintris-on-c64-par...