My printer (a Canon Pro 1000) only allows borderless printing for specific sizes.
This makes sense because I understand it needs to know how to bleed over specific width.
But if I set the width to one of the 'standard' ones and the length to some custom value, it still won't let me print borderless. Why not?
I think that is simply negligence on the software engineering part.
I know that you are talking about the Canon Pro 1000, as you said you own that one in a recent answer you gave.
Just to delve a bit into the borderless printing problem and why this seems to be a software problem:
Why is there a problem at all?
Borderless printing has the issue that no printer is precise enough to really stop at the borders, so that ink is sprayed after the printer head as reached and crossed the edge of the paper.
As this ink would start to stain the bottom plate of the printer, the plate includes grooves and channels to catch that overspray and slowly conduct it into the waste ink cartridge (or maintenance cartridge).
As for the start and he end of the paper, there has to be a horizontal groove as well.
As the position of the side edges cannot be influenced by the printer apart fro the paper inserting system, they are fixed and only format of the width where grooves are present will allow for borderless printing.
The position of the start and end of the paper is always the same and under full control of the printer's paper feed system. So there is seemingly no hardware requirement to also limit the length of the paper.
Possible reasons
I can imagine that they would like to restrict borderless printing for paper rolls - as the sheer amount of over-sprayed ink might overwhelm the channel capacity.
Or that they fear that a small angular deviation when inserting a roll of paper (even though this printer has a mechanism to correct that for sheet paper) might lead to a situation where deviation adds up due to the sheer length of paper and the paper then travels too much from the channels. This will lead to the white of the paper becoming visible, ruining the print.
However the restriction is also in place for shorter paper formats. Which is puzzling.
My conclusion
So if we are not overlooking something: The reason why your printer does not allow borderless printing is probably just inflexible firmware/drivers on the printer side - or disallowing situations where the print has a greater chance of being ruined by small imprecisions of the printing process.
